Topic 3 Genetics Flashcards

Question

Outline what happens to the zygote. ## Footnote 3.2

Answer 1

* **When the zygote divides by mitosis, more cells with diploid nuclei are produced.** * Many animals and plants consist entirely of diploid cells, apart from the cells that they are using to produce gametes for sexual reproduction. ## Footnote Understanding: Diploid nuclei have pairs of homologous chromosomes.

Answer 2

**One of the most fundamental characteristics of a species is the number of chromosomes.** * Organisms with a different number of chromosomes are unlikely to be able to interbreed so all the interbreeding members of a species need to have the same number of chromosomes. ## Footnote Understading: The number of chromosomes is a characteristic feature of members of a species.

Answer 3

Parascaris equorum (horse) - **4** Oryza sativa (rice) - **24** Homo sapiens - **46** Pan troglodytes (chimpanzees) - **48** Cannis familiaris (dog) - **78** ## Footnote Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.

Answer 4

There are two chromosomes in humans that determine sex: * the X chromosome is relatively large and has its centromere near the middle. * the Y chromosome is much smaller and has its centromere near the end. ## Footnote Understanding: Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.

Answer 5

* Females possess two copies of a large X chromosome (XX) * Males possess one copy of an X chromosome and one copy of a much shorter Y chromosome (XY) ## Footnote Understanding: Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.

Answer 6

* Because the X and Y chromosomes determine sex they are called the sex chromosomes. * All the other chromosomes are autosomes and do not affect whether a fetus develops as a male or female. ## Footnote Understanding: Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.

Answer 7

The chromosomes of an organism are arranged into homologous pairs according to size in decreasing length (with sex chromosomes shown last) ## Footnote Understanding: A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.

Answer 8

A karyotype is a property of an organism * it is the number and type of chromosomes that the organism has in its nuclei. ## Footnote Application: Use of karyotypes to deduce sex and diagnose Down syndrome in humans.

Answer 9

1. To deduce whether an individual is male or female. **If two XX chromosomes are present the individual is female whereas one X and one Y indicate a male.** 2. To diagnose Down syndrome and other chromosome abnormalities. **If there are three copies of chromosome 21 in the karyotype instead of two**, the child has Down syndrome. This is sometimes called trisomy 21. ## Footnote Application: Use of karyotypes to deduce sex and diagnose Down syndrome in humans.

Answer 10

The process of meiosis consists of two cellular divisions: 1. The first meiotic division separates pairs of homologous chromosomes to halve the chromosome number (diploid → haploid) 2. The second meiotic division separates sister chromatids (created by the replication of DNA during interphase) ## Footnote Understanding: One diploid nucleus divides by meiosis to produce four haploid nuclei.

Answer 11

* In eukaryotic organisms, **sexual reproduction involves the process of fertilization.** * Fertilization is the union of sex cells, or gametes, from two different parents. * Fertilization **doubles the number of chromosomes each time it occurs. (therefore halving of chromosome number occurs in meioisis creating gamtes).** * Fertilisation of two haploid gametes (egg + sperm) will result in the **formation of a diploid zygote that can grow via mitosis** ## Footnote Understanding: The halving of the chromosome number allows a sexual life cycle with fusion of gametes.

Answer 12

* DNA in the nucleus is replicated during the interphase before meiosis, so **each chromosome consists of two sister chromatids**. ## Footnote Understanding: DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.

Answer 13

1. Firstly **homologous chromosomes pair up with each other.** A pair of homologous chromosomes is bivalent and the pairing process is sometimes called synapsis. 2. Soon after synapsis, a process called **crossing over takes place.** A junction is created where one chromatid in each of the homologous chromosomes breaks and rejoins with the other chromatid. 3. **Crossing over occurs at random positions** anywhere along the chromosomes. 4. Because a crossover occurs at precisely the same position on the two chromatids involved, there is a **mutual exchange of genes between the chromatids.** 5. As the chromatids are homologous but not identical, some alleles of the exchanged genes are likely to be different. **Chromatids with new combinations of alleles are therefore produced.** ## Footnote Understanding: The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.

Answer 14

* Each chromosome is attached to one pole only, not to both. * **The two homologous chromosomes in a bivalent are attached to different poles.** * The pole to which each chromosome is attached depends on which way the pair of chromosomes is facing. This is called the orientation. * The **orientation of bivalents is random**, so each chromosome has an equal chance of attaching to each pole, and eventually of being pulled to it. ## Footnote Understanding: Orientation of pairs of homologous chromosomes prior to separation is random.

Answer 15

* Initially the two chromosomes in each bivalent are held together by chiasmata * When homologous chromosomes are seperated and no longer held by chiamsata it is called disjunction. * One chromosome from each bivalent moves to one of the poles and the other chromosome to the other pole. * **Seperation of pairs of homologous chromosomes to opposite poles of the cell halves the chromosome number of the cell.** ## Footnote Understanding: Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.

Answer 16

Amniocentesis involves passing a needle through the mother's abdomen wall, using ultrasound to guide the needle. * The needle is used to withdraw a sample of amniotic luid containing fetal cells from the amniotic sac. ## Footnote Application: Methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.

Answer 17

A sampling tool that enters through the vagina is used to obtain cells from the chorion, one of the membranes from which the placenta develops. ## Footnote Application: Methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.

Answer 18

* Amniocentesis * chorionic villus sampling * chorionic villus sampling can be done earlier in the pregnancy than amniocentesis, but whereas the risk of miscarriage with **amniocentesis is 1%, with chorionic villus sampling it is 2%.** ## Footnote Application: Methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.

Answer 19

**Prophase I** * Cell has 2n chromosomes (double chromatid): n is haploid number of chromosomes. * **Homologous chromosomes pair (synapsis).** * **Crossing over occurs.** **Metaphase I** * Spindle microtubules move homologous pairs to equator of cell. * **Orientation** of paternal and maternal chromosomes on either side of equator is **random** and independent of other homologous pairs. **Anaphase I** * Homologous pairs are separated. One chromosome of each pair moves to each pole. **Telophase I** * Chromosomes uncoil. During interphase that follows, **no replication occurs.** * Reduction of chromosome number from **diploid to haploid** completed. * Cytokinesis occurs. ## Footnote Skill: Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.

Answer 20

1. **Random orientation of bivalents** (In metaphase I) 2. **Crossing over** (allows linked genes to be reshuffled, to produce new combinations) ## Footnote Understanding: Crossing over and random orientation promotes genetic variation.

Answer 21

* It allows alleles from two different individuals to be combined in one new individual. ## Footnote Understanding: Fusion of gametes from diferent parents promotes genetic variation.

Answer 22

When homologous chromosomes fail to separate at anaphase. ## Footnote Application: Non-disjunction can cause Down syndrome and other chromosome abnormalities.

Answer 23

* Both of the chromosomes move to one pole and neither to the other pole. * **The result will be a gamete that either has an extra chromosome or is defcient in a chromosome.** * If the gamete is involved in human fertilization, the result will be an individual with either 45 or 47 chromosomes. ## Footnote Application: Non-disjunction can cause Down syndrome and other chromosome abnormalities.

Answer 24

* **Trisomy 21**, also known as Down syndrome: individual has three of chromosome number 21 instead of two. * **Trisomy 18 (Edwards Syndrome) and trisomy 13 (Patau’s Syndrome)** * **Abnormal numbers of sex chromosomes** (ex. Klinefelter's syndrome is caused by having the sex chromosomes XXY. Turner's syndrome is caused by having only one sex chromosome, an X.) ## Footnote Application: Non-disjunction can cause Down syndrome and other chromosome abnormalities.

Answer 25

Studies show that the chances of non-disjunction increase as the age of the parents increase * There is a particularly strong correlation between maternal age and the occurrence of non-disjunction events ## Footnote Application: Studies showing age of parents infuences chances of non-disjunction

Answer 26

Gregor Mendel developed the principles of inheritance by performing experiments on pea plants. * First, he crossed different varieties of purebred pea plants, then collected and grew the seeds to determine their characteristics * Next, he crossed the offspring with each other (self-fertilization) and grew their seeds to similarly determine their characteristics * These crosses were performed many times to establish reliable data trends (over 5,000 crosses were performed) ## Footnote Understanding: Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.

Answer 27

Gametes contain one chromosome of each type so are haploid. * **The nucleus of a gamete therefore only has one allele of each gene.** ## Footnote Understanding: Gametes are haploid so contain one allele of each gene.

Answer 28

The nucleus of the zygote contains two chromosomes of each type so is diploid. It contains also two alleles of each gene. ## Footnote Understanding: Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or diferent alleles.

Answer 29

* During meiosis I, **homologous chromosomes are separated into different nuclei prior to cell division** * As homologous chromosomes carry the same genes, **segregation of the chromosomes also separates the allele pairs** * Consequently, as gametes contain only one copy of each chromosome they therefore carry **only one allele of each gene** * It breaks up existing combinations of alleles in a parent and **allows new combinations to form** in the offspring. ## Footnote Understanding: The two alleles of each gene separate into diferent haploid daughter nuclei during meiosis.

Answer 30

The gene composition (i.e. allele combination) for a specific trait ## Footnote Understanding: Dominant alleles mask the efects of recessive alleles but co-dominant alleles have joint efects.

Answer 31

Most traits follow a classical dominant / recessive pattern of inheritance, whereby one allele is expressed over the other * **The dominant allele will mask the recessive allele when in a heterozygous state** * Homozygous dominant and heterozygous forms will be phenotypically indistinguishable * **The recessive allele will only be expressed in the phenotype when in a homozygous state** When representing alleles, the convention is to capitalise the dominant allele and use a lower case letter for the recessive allele * ex. BB or Bb or bb ## Footnote Understanding: Dominant alleles mask the efects of recessive alleles but co-dominant alleles have joint efects.

Answer 32

**Co-dominance occurs when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual** * Heterozygotes therefore have an altered phenotype as the alleles are having a joint effect When representing alleles, the convention is to use superscripts for the different co-dominant alleles (recessive still lower case) * ex. C^W ## Footnote Understanding: Dominant alleles mask the efects of recessive alleles but co-dominant alleles have joint efects.

Answer 33

* Homozygous: two of the same allele * Heterozygous: two different alleles. ## Footnote Understanding: Dominant alleles mask the efects of recessive alleles but co-dominant alleles have joint efects.

Answer 34

* The A and B alleles are co-dominant and each modify the structure of the antigen to produce different variants * The O allele is recessive and does not modify the basic antigenic structure **A allele = IA ; B allele = IB ; O allele = i (recessive)** ## Footnote Application: Inheritance of ABO blood groups.

Answer 35

**A genetic disease is an illness that is caused by a gene. **Most genetic diseases are caused by a recessive allele og a gene. * The disease therefore only develops in individuals that do not have the dominant allele of the gene, usually because they have **two copies of the recessive allele.** ## Footnote Understanding: Many genetic diseases in humans are due to recessive alleles of autosomal genes.

Answer 36

* **If a person has one allele for the genetic disease and one dominant allele**, they will not show symptoms of the disease, but they can pass on the recessive allele to their offspring. These individuals are called carriers. ## Footnote Understanding: Many genetic diseases in humans are due to recessive alleles of autosomal genes.

Answer 37

* A dominant allele (ex. Huntington's disease) * Co-dominant alleles (ex. sickle-cell anemia) * sex linkage (ex. colour blindness and hemophillia) ## Footnote Understanding: Some genetic diseases are sex-linked and some are due to dominant or co-dominant alleles.

Answer 38

**Cystic fibrosis is an autosomal recessive disorder caused by a mutation to the CFTR gene on chromosome 7** * Individuals with cystic fibrosis produce mucus which is unusually thick and sticky * This mucus clogs the airways and secretory ducts of the digestive system, leading to respiratory failure and pancreatic cysts * Heterozygous carriers who possess one normal allele will not develop disease symptoms ## Footnote Application: Inheritance of cystic fibrosis and Huntington's disease.

Answer 39

* **Huntington's disease is due to a dominant allele of the HTT gene.** * This gene is located on chromosome 4 and the gene product is a protein named huntingtin. * The dominant allele of HTT causes degenerative changes in the brain. ## Footnote Application: Inheritance of cystic fibrosis and Huntington's disease.

Answer 40

Sex-linked inheritance patterns differ from autosomal patterns due to the fact that the chromosomes aren’t paired in males (XY) * This leads to the expression of sex-linked traits being predominantly associated with a particularly gender As human females have two X chromosomes (and therefore two alleles), they can be either homozygous or heterozygous * Hence, **X-linked dominant traits are more common in females** (as either allele may be dominant and cause disease) Human males have only one X chromosome (and therefore only one allele) and are hemizygous for X-linked traits * **X-linked recessive traits are more common in males**, as the condition cannot be masked by a second allele ## Footnote Understanding: The pattern of inheritance is diferent with sex-linked genes due to their location on sex chromosomes.

Answer 41

* **Only females can be carriers** (a heterozygote for a recessive disease condition), males cannot be heterozygous carriers * **Males will always inherit an X-linked trait from their mother** (they inherit a Y chromosome from their father) * **Females cannot inherit an X-linked recessive condition from an unaffected father** (must receive his dominant allele) ## Footnote Understanding: The pattern of inheritance is diferent with sex-linked genes due to their location on sex chromosomes.

Answer 42

red-green colour-blindness and hemophilia ## Footnote Application: Red-green colour-blindness and hemophilia as examples of sex-linked inheritance.

Answer 43

Caused by a **recessive allele** of a gene for one of the photoreceptor proteins. ## Footnote Application: Red-green colour-blindness and hemophilia as examples of sex-linked inheritance.

Answer 44

Due to an inability to make Factor VIII, one of the proteins involved in the clotting of blood. * The allele that causes hemophilia is **recessive**. ## Footnote Application: Red-green colour-blindness and hemophilia as examples of sex-linked inheritance.

Answer 45

* males are shown as squares * females are shown as circles * squares and circles are shaded or cross-hatched to indicate whether an individual is affected by the disease * Roman numerals indicate generations ## Footnote Skill: Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases.

Answer 46

The reason for this is that most genetic diseases are caused by very rare **recessive alleles** which follow Mendelian patterns of inheritance. * The chance of inheriting one allele for any specifc disease is small but to develop the disease two alleles must be inherited and the chance of this is extremely small. ## Footnote Understanding: Many genetic diseases have been identifed in humans but most are very rare.

Answer 47

The reason for this is that most genetic diseases are caused by very rare **recessive alleles** which follow Mendelian patterns of inheritance. * The chance of inheriting one allele for any specifc disease is small but to develop the disease two alleles must be inherited and the chance of this is extremely small. ## Footnote Understanding: Many genetic diseases have been identifed in humans but most are very rare.

Answer 48

**A mutation is a random change to the base sequence of a gene. Two types of factor can increase the mutation rate.** 1. **Radiation** (Gamma rays, short-wave ultraviolet radiation and X-rays are all mutagenic.) 2. **Chemical substances** (ex. mustard gas) ## Footnote Understanding: Radiation and mutagenic chemicals increase the mutation rate and can cause genetic disease and cancer.

Answer 49

Apart from cancer the other main effect of the radiation that was predicted was mutations, leading to stillbirths, malformation or death. ## Footnote Application: Consequences of radiation after nuclear bombing of Hiroshima and Nagasaki and the nuclear accidents at Chernobyl.

Answer 50

An explosion at the reactor core caused the release of radioactive material. * thyroid cancer * Bioaccumulation caused high levels of radioactive caesium in fish ## Footnote Application: Consequences of radiation after nuclear bombing of Hiroshima and Nagasaki and the nuclear accidents at Chernobyl.

Answer 51

Gel electrophresis is a method of seperating mixtures of proteins or fragments of DNA, which are charged. 1. The mixture of DNA is placed on a thin sheet of gel, which acts like a molecular sieve. 2. An electric field is applied to the gel by attatching electrodes to both ends. 3. Depending on whether the particles are positively or negatively charged, they move towards one of the electrodes or the other. 4. The rate of movement depends on the size of the molecules (small molecules move faster than larger ones). ## Footnote Understanding: Gel electrophoresis is used to separate proteins or fragments of DNA according to size.

Answer 52

Can rapidly copy small samples of DNA into larger amounts needed for analysis. 1. **Denaturation:** DNA sample is heated to separate it into two strands (~95 deg C) 2. **Annealing:** DNA primers attach to opposite ends of the target sequence (~50 deg C) 3. **Elongation:** A heat-tolerant DNA polymerase (Taq) copies the strands (~72 deg C) ## Footnote Understanding: PCR can be used to amplify small amounts of DNA.

Answer 53

1. **A sample of DNA is obtained**, either from a known individual or from another source such as a fossil or a crime scene. 2. **Sequences in the DNA** that vary considerably between individuals are selected and are **copied by PCR.** 3. The copied **DNA is split into fragments using restriction endonucleases.** 4. The fragments are separated using **gel electrophoresis.** 5. **This produces a pattern of bands** that is always the same with DNA taken from one individual. This is the individual's DNA profle. 6. The profiles of different individuals can be compared to see which bands are the same and which are different. ## Footnote Understanding: DNA profling involves comparison of DNA.

Answer 54

**Forensic investigations** 1. DNA profle of material from the crime scene is compared with the DNA profile of a sample of DNA taken grom the suspect or the victim. 2. If the pattern of bands matches exactly it is highly likely that the two samples of DNA are from the same person. 3. This can provide very strong evidence o who committed the crime. **Paternity investigations** 1. DNA profiles of the mother, the child and the man are needed. 2. DNA profiles of each of the samples are prepared and the patterns of bands are compared. 3. If any bands in the child's profle do not occur in the profle of the mother or man, another person must be the father. 4. Children inherit half their chromosomes from each parent and thus should possess a combination of parental fragments ## Footnote Application: Use of DNA profling in paternity and forensic investigations.

Answer 55

A gene determines a particular trait by encoding for a specific polypeptide in a given organism * Because the genetic code is universal, an organism can potentially express a new trait if the appropriate gene is introduced into its genome * The transfer of genes between species is called gene modification, and the new organism created is called a transgenic ## Footnote Understanding: Genetic modifcation is carried out by gene transfer between species.

Answer 56

1. **Restriction enzymes ‘cut’ the desired gene from the genome. ** 2. E. coli bacteria contain small circles of DNA called plasmids. 3. The **same restriction enzyme cuts into the plasmid.** 4. Because it is the same restriction enzyme the same bases are left exposed, creating **‘sticky ends’** 5. **Ligase joins the sticky ends**, using complementary base pairing, **fixing the gene into the E. coli plasmid.** 6. The **recombinant plasmid is inserted into the host cell**. It now expresses the new gene. An example of this is human insulin production. 7. Fermenters are used to produce large quantities of bacteria. The **human insulin is then separated from the bacteria and purified.** ## Footnote Application: Gene transfer to bacteria with plasmids using restriction endonucleases and DNA ligase.

Answer 57

1. Higher crop yield, more food, less pest damage 2. Less land needed for crop production 3. Less use of insecticides, expensive and harmful ## Footnote Application: Assessment of the potential risks and benefts associated with genetic modifcation of crops.

Answer 58

1. Insects that are not pests are killed (ex. toxin in Bt maize harms monarch butterflies) 2. Transfered gene may spread to population of other plants by cross pollination, makes them also toxic to insects 3. Insects pests may develop resistance to toxin, even harder to control afterwards ## Footnote Application: Assessment of the potential risks and benefts associated with genetic modifcation of crops.

Answer 59

**The production of genetically identical organisms** is called cloning and a group of genetically identical organisms is called a clone. ## Footnote Understanding: Clones are groups of genetically identical organisms, derived from a single original parent cell.

Answer 60

**The production of genetically identical organisms** is called cloning and a group of genetically identical organisms is called a clone. ## Footnote Understanding: Clones are groups of genetically identical organisms, derived from a single original parent cell.

Answer 61

Uses asexual reproduction. * extra bulbs * tubers * runners ## Footnote Understanding: Many plant species and some animal species have natural methods of cloning.

Answer 62

An embryo divides into two or more parts and each part to develop into a separate individual with all body parts. ## Footnote Understanding: Animals can be cloned at the embryo stage by breaking up the embryo into more than one group of cells.

Answer 63

Formation of identical twins could be regarded as cloning by splitting. ## Footnote Understanding: Animals can be cloned at the embryo stage by breaking up the embryo into more than one group of cells.

Answer 64

1. Remove a differentiated diploid nucleus from the individual to be cloned. 2. Enucleate a donor egg cell. (remove nucleus) 3. Insert the diploid nucleus into the enucleated egg cell. 4. Implant into the endometrium of a surrogate mother and gestate. 5. The newborn will be genetically identical to the donor nucleus parent. ## Footnote Understanding: Methods have been developed for cloning adult animals using diferentiated cells.

Answer 65

1. Remove a differentiated diploid nucleus from the cell to be cloned. 2. Enucleate a donor egg cell. 3. Insert the diploid nucleus into the enucleated egg cell. 4. Stimulate it to divide and grow in vitro. 5. The resulting embryo is a rich source of stem cells which can be harvested or cultured. 6. The outer layer of cells is removed, so only the inner cell mass is used to culture the tissues needed. ## Footnote Understanding: Methods have been developed for cloning adult animals using diferentiated cells.

Answer 66

Creating an embryo as a source of stem cells, by transfer of a differentiated nucleus. ## Footnote Understanding: Methods have been developed for cloning adult animals using diferentiated cells.

Answer 67

Creating a genetically identical organism through transfer of a differentiated diploid nucleus. ## Footnote Understanding: Methods have been developed for cloning adult animals using diferentiated cells.

Topic 3 Genetics Flashcards

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