Inheritance

Question 1

What did Mendel do? What were his findings, also describe what punnet square analysis showed.

Answer 1

-Crossed Pure bred homozygous plants to understand inheritance
> crossed 1 tall, 1 short plant - tall phenotype = dominant
> Cross bred the offspring , 3;1 ratio of tall; short
> When Punnett square analysis was carried out there were 3 possible genotypes, however these result in only 2 phenotypes.

Question 2

What were Mendels 2 laws?

Answer 2

1) First Law- Law of segregation
> Each individual possesses two “units of inheritance” (i.e. genes) for each characteristic, although only one of these is transmitted to each offspring.

2) Second Law- Law of independent assortment
> Genes at different loci segregate independently e.g. flower colour and plant height units of inheritance moved independently.

Question 3

What patterns of inheritance is there?

Answer 3

• Autosomal dominant : Gene on autosome (1-22), Single dominant allele results in the associated phenotype (the trait will be expressed in both homozygous and heterozygous genotypes)… trait usually heterozygous
• Autosomal recessive: Phenotype only results when there are two copies of the recessive allele - both parents must (at least) be carriers… More common in smaller populations
• X-linked dominant: Phenotype is observed in females with a single allele, Males are often more severely affected than females.
• X-linked recessive: In males, phenotype results from the presence of a single allele , In females, phenotype results from the presence of two alleles.
  > Males are either affected or unaffected but females with a single allele are unaffected carriers
• Y- linked: Genes located on the Y chromosome, Father to all his sons but never daughters.. Holandric
• Co-dominant: Two dominant alleles are expressed equally referred to as co-dominance

Question 4

Genetic pedigree symbols…

Answer 4

Question 5

1. What is the structure of a chromosomes?
2. We are D….. organisms ? What does this mean?

Answer 5

1- Chromatin Long strands of DNA wound around structural proteins called histone

2- Diploid: 2 copies of each chromosome one from mama and baba
> 46 chromosomes .. 23 pairs.. 22 are autosomal

Question 6

What is the difference between chromosomes and sister chromatids?

Answer 6

• Chromosomes refers to an un-replicated chromosome that is within the nucleus in a diffuse (uncoiled) state.
• Chromosomes which are seen after mitosis or meiosis- at this point they are referred to as chromatids, a pair of those are referred to as sister chromatids

Question 7

What is a Karyotype?
-The two similar versions of each chromosome (one paternal and one maternal) are referred to as ????

Answer 7

• Individual’s collection of chromosomes.
  > Each DIPLOID human cell nucleus contains two similar versions of each autosome plus a pair of sex chromosomes (2 x 22 + 2= 46 chromosomes in total)

homologous chromosomes or homologs

Question 8

What is the process to create a karyotype?

Answer 8

• Chromosomes are shown stained with Giemsa stain and viewed under a microscope.
• Image adjusted to show single (not replicated) copy of each homolog pair- 1 maternal and 1 paternal.
• They are lined up in size- as chromosome number increases the autosomes become smaller.

Question 9

What is the point of Mitosis?

Answer 9

> Increase cell number – to grow, or to repair or replace damaged tissue.

> produce 2 genetically identical diploid daughter cells with a full complement of chromosomes (46 in human cells) with exactly the same genetic composition as the original cell.

Question 10

What are the 3 reasons we need meiosis?

Answer 10

• Gamete production for sexual reproduction
• Haploid daughter cells with 1/2 amount of genetic material
• Introduce genetic diversity

Question 11

What happens in Meiosis I?
2n 2c =
2n= diploid
2c= 2 of each chromosome

Answer 11

1 - DNA replicated at S-phase (same as Mitosis!)
2- Undergoes prophase, metaphase, anaphase, telophase and cytokinesis (same basic stages as mitosis)
3- At prophase I the duplicated homologous pairs line up and regions of the chromosomes cross over to exchange genetic material (chiasma). Crossing over occurs between the homolog pairs, not the sister chromatids, as these are genetically identical.
- At anaphase I the duplicated homologous pairs segregate (as opposed to sister chromatids in Mitosis)

> The result is two, HAPLOID daughter cells. They are haploid because the 2 chromosomes are identical to each (either duplicated maternal or paternal homologs

Question 12

What happens in Meiosis II?

Answer 12

1- No duplication of DNA, cells are still HAPLOID
2- Undergoes a second round of cell division
3- At anaphase II the sister chromatids are pulled apart (as in Mitosis)
4- Cytokinesis occurs

> The result is four, HAPLOID daughter cells that are genetically different from one another and different from the haploid cells that formed the organism in the first place

Question 13

Describe Independent assortment 1 of the processes in which genetic variation occurs?

Answer 13

• Genetically different from each other and the haploid cells that formed the parent organism arise
• Each haploid cell (gamete) contains one chromosome from each of the homolog pairs (either the maternal or paternal homolog), the maternal and paternal chromosomes are shuffled into novel combinations.
  1- It occurs during anaphase I. Homolog pairs align on the spindle in a random arrangement – so whether you get maternal or paternal on each side of the spindle is a complete coincidence.
• When the cell divides that is when you get random shuffling of maternal and paternal homologs
• 23 homolog pairs can form around 8 million different gametes by the end of Meiosis II and this is BEFORE crossing over between homolog pairs, which gives arise to even further variation.

Question 14

Describe Crossing over 1 of the processes in which genetic variation occurs?

Answer 14

• homologous maternal and paternal chromosomes exchange genetic information during prophase 1.
• Crossing over does not occur between sister chromatids, only the homolog pairs

Question 15

What happens if meiosis goes wrong?

Answer 15

• Improper separation of homologs is referred to as nondisjunction – leads to
  chromosomal abnormalities.
  > More common during female meiosis which arrests for years following diplotene
  of Prophase I
  > Chromosomal segregation errors during egg development are the most common cause of spontaneous abortion (miscarriage).

Question 16

What happens in Non-disjunction?

Answer 16

• In meiosis I nondisjunction occurs between homolog pairs
  → All gametes effected as a result
• In meiosis II nondisjunction occurs between sister chromatids
  → Half gametes affected

Question 17

• Cells with an abnormal number of chromosomes are referred to as….

Answer 17

• aneuploid - abnormal
• euploid - normal

Question 18

• Aneuploid gametes form abnormal embryos that…
  What is Down syndrome an example of?

Answer 18

• spontaneously abort
  > Results in an additional copy of chromosome 21 (trisomy 21 – 3 copies of chromosome 21)

Question 19

What causes of Down Syndrome are there?

Answer 19

Mosaic trisomy is an idea that within an embryo, some cells have an extra copy but others are unaffected. It is a very rare form.

Question 20

Risk of having a baby with Down syndrome dramatically increases with increasing maternal age- why?

Answer 20

• Inherent consequence of the process through which oocytes are formed
  >Oogenesis begins within a foetus at around 5-6 weeks gestation. It then then arrests in Prophase I – this process is not completed until the egg is released from the ovary as part of the menstrual cycle- somewhere between 15 and 45 years later!!
  > chromosomes become more tightly bound to each other over time, and lose lose their ability to segregate from each other.