Mechanisms of Hereditary

Question 1

What 3 questions did Mendel want to answer?

Answer 1

1. What is inherited?
2. How is it inherited?
3. What is the role of chance?

Question 2

Blended Inheritance

Answer 2

Information from parents is irreversibly linked, there is no way for it to be separated out again.

Question 3

Discrete Inheritance

Answer 3

Information from parents is passed down and maintained separately in an individual

Question 4

Monohybrid Cross

Answer 4

Cross between 2 individuals that differ in only 1 trait

Question 5

True-Breeders Definition

Answer 5

Homozygous organisms who’s phenotype is always passed on to their offspring

Question 6

Advantages of using Pea plants

Answer 6

• Short generation time (1 yr per offspring)
• Individuals produce lots of offspring
• Hermaphroditic (i.e. can self-fertilise)
• Clear-cut binary qualitative traits (e.g. yellow OR green)

Question 7

Reciprocal Crosses

Answer 7

2 Monohybrid crosses with the same trait being investigate, but the parent phenotypes switch in the second cross (i.e. if dad was purple in first cross, mom will be purple in second cross)

Question 8

Results of Mendel’s first experiments

Answer 8

• In F2 gen, sometimes 1 of the parental phenotypes that disappeared would reappear
• Reappeared in ration of ±1:3
• This disproved blended inheritance

Question 9

Conclusions from Mendel’s first experiments

Answer 9

1. Some phenotypes are more dominant over others
2. 2 units of inheritance in every trait in an individual - 1 from father, 1 from mother. These are maintained in the individual.

Question 10

Mendels 1st Law

Answer 10

Principle of Segregation

• The 2 alleles for each trait segregate during gamete formation and then unite randomly at fertilisation (1 from each parent)

Question 11

The Product Rule

Answer 11

When event are completely independent of each other (gamete formation)

• Probability of 2 independent event occurring together is the product of the probability that each event will occur on its own
  i. e. prob of throwing 2 6’s in a row = 1/6 x1/6 = 1/36

Question 12

The Sum Rule

Answer 12

Phenotypic classes (gamete fusion)

• Probability of 2 mutually exclusive events occurring is the sum of their individual probabilities
  i. e. prob of throwing a 5 OR a 6 in a single dice roll: 1/6 +1/6 = 2/6

Question 13

Test Cross

Answer 13

Cross an individual with an unclear genotype with a homozygous recessive individual

• If all F1 gen are dominant phenotypes then mystery individual must by homozygous dominant
• If 50-50 then must be heterozygous

Question 14

Dihybrid Crosses

Answer 14

Crosses with 2 different traits

Question 15

Recombinant phenotypes

Answer 15

New combinations of phenotypes that don’t correspond to either of the parents

Question 16

Mendels 2nd Law

Answer 16

Independent assortment

During gamete formation, different pairs of alleles segregate independently of one another

Question 17

Chi-squared test

Answer 17

• Goodness of fit test
• Can be used to determine whether results we observe in a cross are consistent with Mendelian inheritance
• test statistic:
  x^2 = sum [(O-E)^2/E]
• Larger x-squared is, the more your data deviates from expected pattern
• reject null hypothesis if p-val(from table) <=0,05

Question 18

5 Causes of Non-Mendelian Inheritance

Answer 18

1. Dominance relationship between pairs of alleles is not complete
2. Lethality: Some allele combination are unfavourable
3. Interaction between genes - if phenotype is not determined by a single gene
4. Sex-linkage - gene carried on sex chromosome
5. Linkage & Recombination - 2 genes on the same chromosome

Question 19

Incomplete Dominance

Answer 19

The inherited phenotype is an intermediate of the 2 parental phenotypes ( brown cow from white and black parents)

Question 20

Co-Dominance

Answer 20

2 alleles are equally dominant over each other. Both parental phenotypes are expressed

Question 21

Non-Mendelian Inheritance: Incomplete Dominance

Answer 21

• Because the dominant phenotype is expressed in homo/heterozygous genotypes, 1 allele is usually the same as 2 (i.e. both states produce enough enzyme to fulfil its function)
• In cases of incomplete dominance, 1 allele is no longer the same as having 2. It doesn’t quite produce the same amount of enzyme/function as a homozygous individual would.

NB: There is no deviation from Mendelian genetics, it is just expressed differently!

Question 22

Non-Mendelian Inheritance: Lethality

Answer 22

• If you get a 2:1 ratio in F1, then it is lethal to be homozygous for the dominant allele
• If you get all 1 phenotype in F1, then it is lethal to be homozygous for the recessive allele

Question 23

Non-Mendelian Inheritance: Complementary Gene action

Answer 23

• 2 genes work together to control the outcome.

- 9:7 ratio is proof of complementary gene action

Question 24

Complementation Tests

Answer 24

• Determine whether 2 mutants (with the same phenotype) are defective in the SAME gene or in DIFFERENT genes.
• Cross 2 mutants together, if the mutant phenotype is present in F1 then they are defective in the SAME gene
• If you cross 2 mutants together and see wild-type phenotype in F1, then they are defective on DIFFERENT genes

Question 25

Complementation Groups

Answer 25

Mutants that are defective in the SAME gene | i.e. they DON'T complement each other

Question 26

Non-Mendelian Inheritance: Recessive Epistasis

Answer 26

- 'Epistasis' = interaction - Homozygosity for a recessive allele at 1 locus masks genotype at a second locus - Ratio 9:3:4 e. g. 2 pigment alleles for fur color in dogs (black & brown), different gene regulates deposition. If homozygous recessive for deposition then pigment is produced but can’t be deposited = yellow fur.

Question 27

Non-Mendelian Inheritance: Dominant Epistasis

Answer 27

- Dominant alleles at one locus masks genotype at a second locus - Ratio 12:3:1

Question 28

Non-Mendelian Inheritance: Chromosomal Theory of Inheritance

Answer 28

- There are 2 copies of each chromosome in every cell & 2 copies of each gene - During Meiosis homologous chromosomes line up and separate to different gametes (obeys the laws of segregation) - This process is independent of the separation of other homologous chromosomes (law of independent assortment)

Question 29

Hemizygous Individual

Answer 29

Male with a gene carried on the X chromosome

Question 30

Non-Mendelian Inheritance: Linkage & Recombination

Answer 30

- 2 genes that are on the same chromosome won't segregate in independent assortment - To test for linkage: do a test cross, if all 4 phenotypes are there in equal numbers then they assorted independently - There will be much more parental phenotypes if the genes are on the same chromosome

Question 31

Why are humans terrible organisms. for genetic analysis?

Answer 31

- Few offspring - Long generation - Resource intensive - Ethics - controlled crosses - Can't self-fertilise

Question 32

What is a pedigree analysis?

Answer 32

Looking back in time at phenotypic data to determine the nature of human genetic disorders.

Question 33

Dominant vs Recessive in Pedigree Analysis

Answer 33

- If a disorder skips a generation then it is recessive. | - If a disorder is dominant it will be in every generation

Question 34

Autosomal vs X-linked disorders in Pedigree Analysis

Answer 34

- If it affects males and females equally it is autosomal - If a diseased male passes on the disease to his son then it HAS to be autosomal (Father doesn't give X-chromosome to son, so can't be x-linked)

Question 35

Dosage Compensation Definition

Answer 35

The equalisation of X-linked gene products in the 2 sexes

Question 36

Strategies for Dosage Compensation

Answer 36

1. Hyper activation of X in males. Increased transcription from X chromosome (happens in fruit flies). 2. Random inactivation of 1 X chromosome. - Happens in mammals - Means that a recessive phenotype might be expressed in a female because the dominant allele could be shut off.

Question 37

How is dosage compensation achieved?

Answer 37

Through large scale modification of chromatin structure (chromatin remodelling). Nucleosome positioning and chromatin condensed structure blocks transcription factors from binding to DNA

Question 38

Sex Determination in Mammals

Answer 38

- Y chromosome determines sex. - Default is for undifferentiated gonad tissue to differentiate into ovarian tissue. - SRY (sex determining region of Y) gene must be present for it to differentiate into teste tissue.