INHERITANCE

Question 1

Gene

Answer 1

unit of inheritance located at a particular locus of a chromosome with a specific DNA nucleotide sequence

Question 2

Locus

Answer 2

specific location of a gene on a chromosome

Question 3

Allele

Answer 3

alternative form of a gene at a particular gene locus

Question 4

What is a dominant allele

Answer 4

expresses the associated phenotype in both homozygous and heterozygous conditions as it masks the influence of the recessive allele

Question 5

What is a recessive allele

Answer 5

expresses the associated phenotype only in homozygous conditions

Question 6

Genotype (important defi)

Answer 6

complete genetic makeup/allelic composition of an organism

Question 7

Phenotype (important defi)

Answer 7

physical manifestation of a genetic trait that results from a specific genotype and its interaction with the environment

Question 8

Link between genotype and phenotype

Answer 8

• central dogma of molecular biology states that information flows from DNA to RNA then to polypeptide chains
• through gene experssion, different alleles are transcribed into different mRNA and then translated into different polypeptide chains which will give rise to different phenotypes

Question 9

Homozygous

Answer 9

condition in which alleles of a gene pair in diploid condition are identical

• known as homozygote
• called true/pure breeding

Question 10

Heterozygous

Answer 10

condition in which alleles of a gene pair in diploid condition are different

• known as heterozygote

Question 11

True-breeding/pure-breeding

Answer 11

organism that gives rise to all offspring of the same phenotype

Question 12

Carrier

Answer 12

person or other organism that has inherited a recessive allele for a genetic trait or mutation but does not display that trait or show symptoms of the disease

Question 13

What is monohybrid inheritance

Answer 13

inheritance of a single character of contrasting traits that is controlled by a single gene

Question 14

What are Mendel’s 2 postulates and explanations

Answer 14

1. Genetic characters are controlled by alleles existing in pairs
   - a specific allele exists for each trait respectively
   - each diploid individual receives one allele from each parent
2. When 2 unlike alleles responsible for a single character are present in a single individual, one allele is dominant to the other which is said to be recessive
   - heterozygous F1 generation, trait tall stem expressed in heterozygotes as a result of presence of dominant allele
   - in F1 generation, recessive allele masked by the dominant allele
   - recessive allele expressed in absence of dominant allele in homozygous recessive F2 plants

Question 15

Mendel’s First Law of segregation

Answer 15

During the formation of gametes, the paired alleles segregate randomly so that each gamete receives one or the other with equal likelihood

Explanation:
- alleles occur in pairs, each being located on one of the pair of homologous chromosomes
- pair of homologous chromosomes segregate during anaphase I of meiosis, take their alleles with them, resulting in each gamete receiving only one of each type of chromosome

Question 16

What is a reciprocal cross and what is its purpose

Answer 16

sexes of parents are reversed regarding a specific trait

eg. one cross of red-eyed female fruit fly and a white-eyed male, all F1 offspring red-eyed
reciprocal cross of white-eyed female, red-eyed male, female offspring red-eyed, male-offspring white-eyed -> shows sex-link inheritance, males’ phenotype always the same as the mothers

Purpose: determine if inheritance of the trait is sex-linked

Question 17

What is a test cross and what is its purpose

Answer 17

crossing the organism of the dominant phenotype, unknown genotype and homozygous recessive indivdual with homozygous recessive genotype

purpose: determine genotype of individual with dominant phenotype
- Homozygous dominant: all offspring expresses dominant phenotype, inherits one dominant allele from parent A and one recessive allele from parent B, all will be heterozygous, only dominant allele expressed and effects of recessive allele masked, all will have dominant phenotype
- Heterozygous: offspring half expresses dominant phenotype, half expresses recessive phenotype in a 1:1 ratio

Question 18

What is codominance
Explain the Biochemical basis of codominance

Answer 18

a phenomenon in which both alleles are equally exprssed in the phenotype of the heterozygote, heterozygote expresses the phenotypes of both types of homozygotes

both alleles code for functional products, both products appear in the phenotype of the heterozygote, therefore teh heterozygote epxresses the phenotype of both types of homozygotes

Question 19

What are lethal genes

Answer 19

mutations which lead to a non-functional gene that results in lethality, indicating that the affected gene has a fundamental function in the growth, development and survival of an organism

Question 20

Why are mutations usually recessive

Answer 20

1. mutations lead to loss of function of protein
2. in diploid organism, a single functioning copy will result in production of sufficient functional proteins to mask the loss of function allele

Question 21

Explain the biochemical effect of recessive lethal genes in agouti mice

Answer 21

Mutant yellow allele Y dominant to wild-type agouti allele y

Leads to offspring ration 2 yellow fur: 1 agouti fur, atypical ratio due to fetal death of homozygous yellow (YY) coat mice, YY genotype missing from the progency

Question 22

Explain why I^A, I^B is dominant over the i/I^O allele in ABO blood group in humans

Is an example of multiple alleles, when gene has more than 2 alleles

Answer 22

1. Both I^A and I^B code for production of anitgen A and B on surface of red blood cell (RBC) respectively
2. i does not code for the production of any anitgen on the surface of RBC
3. In heterozygotic individuals, I^Ai or I^Bi, only I^A or I^B allele will deetermine the blood group of the individual

Question 23

What is dihybrid inheritance

Answer 23

cross involving 2 characters in which parents posses different forms of each character, the 2 traits are controlled by 2 genes located on 2 gene loci on 2 different chromosomes

Question 24

What is Mendel’s second law of Independent Assortment

Answer 24

The segregation of one pair of alleles is independent of the segregation of other pairs

Question 25

What is the parental phenotypes for dihyrid test cross

Answer 25

organism with 2 dominant traits but **unknown genotype** and an individuals that is homozygous recessive for both gene loci

Question 26

Why did Mendel choose to use pees in his experiments

Answer 26

1. plants had varieties of distinct characteristics, allows for tracking of traits across generations 2. easy to cultivate, large pool of peas, carry out multiple experiments 3. Relatively short life cycle of one year, matures quickly, allows for many generations to be studied efficiently 4. Peas produced a large number of seeds, generate thousands of offspring, larger sample size, reducing random error because of chance variation, getting more consistent ratios 5. Pollination the pea plant was easy to control, cross-fertilization could be accomplished artificially, able to study the cross of many different combinations of phenotypes 6. Pure breeds available, allowing test crosses to be studied

Question 27

What are sex-linked genes

Answer 27

genes located on the sex chromosomes

Question 28

Examples of X-linked human disorders

Answer 28

Haemophilia, colour blindness, Duchenne muscular dystrophy