Topic 7A: Genetics

Question 1

What is monohybrid inheritance?

Answer 1

• Controlled by a single gene

Question 2

What do monohybrid punnet squares show?

Answer 2

• Likelihood of inheritance of a characteristic

Question 3

What things should you show when doing a punnet square?

Answer 3

• Parental phenotypes, genotypes, gametes
• Offspring genotypes, phenotypes
• Ratio of offspring phenotypes

Question 4

What is the expected ratio of a homozygous dominant and homozygous recesssive?

Answer 4

• All dominant

Question 5

What is the ratio for a heterozygote and a homozygous recessive?

Answer 5

• 1: 1

Question 6

What is the ratio for 2 heterozygotes?

Answer 6

• 3 : 1 –> dominant : recessive

Question 7

When is a test cross used and what does it show?

Answer 7

• Used to find the genotype of an individual displaying the dominant characteristic
• Test by crossing with homozygous recessive
• If some offspring show the recessive characteristic –> parent was heterozygous

Question 8

Why is the actual ratio of offspring not the same as the expected?

Answer 8

• Fertilisation is random

Question 9

How can results be made more reliable?

Answer 9

• Use a larger sample size - more representative of the population

Question 10

What are codominant alleles?

Answer 10

• Neither is recessive
• Both are expressed in the phenotype if heterozygous

Question 11

What are multiple alleles?

Answer 11

• More than 2 alleles present for the same gene
• Some dominant, some recessive

Question 12

What is dihybrid inheritance?

Answer 12

• Inheritance of 2 different genes

Question 13

What gametes would be used from RRYY?

Answer 13

• RY only

Question 14

What are the gametes for RrYY?

Answer 14

• RY, rY

Question 15

What are the gametes for rrYy?

Answer 15

• rY, ry

Question 16

What are the gametes for RrYy?

Answer 16

• RY, Ry, rY, ry

Question 17

What is the ratio for 2 dihybrid heterozygotes?

Answer 17

• 9:3:3:1
• 9 = both dominant
• each 3 = 1 dominant
• 1 = both recessive

Question 18

What is linkage?

Answer 18

• Genes found on the same chromosome

Question 19

What is autosomal linkage?

Answer 19

• Linked genes on a chromosome that is not a sex chromosome

Question 20

How can genes be more closely linked?

Answer 20

• Closer together on chromosome
• Less likely to have crossing over occur

Question 21

Is crossing over common?

Answer 21

• It is very rare so non linked gametes much less common

Question 22

For AaBb where bith dominant and both recessives are linked what are the gametes?

Answer 22

• AB, ab

Question 23

What is sex linkage?

Answer 23

• Genes linked on a sex chromosome

Question 24

Why are sex linked recessive disorders more common in males?

Answer 24

• Males - only have 1 X chromosome –> allele present always expressed
• Females - 2 X chromosomes –> only expressed if homozygous recessive

Question 25

What is epistasis?

Answer 25

- When the allele of one gene affects / masks the expression if the allele of another gene

Question 26

Which stats test is used in genetics and why?

Answer 26

- Chi squared - Used when looking for a difference in the number / frequency of data in categories - Look for difference in observed and expected ratios of phenotypes

Question 27

What is the equation?

Answer 27

X2 = ∑▒〖(O-E)〗^2/E - O = observed frequency - E = Expected frequency

Question 28

What are the degrees of freedom? (for chi2)

Answer 28

- Number of categories - 1

Question 29

What do the null hypotheses look like? (for chi2)

Answer 29

- There is no significant difference in the observed and expected numbers in the different categories (phenotypes)

Question 30

What do you want the outcome to be? (for chi2 in genetics)

Answer 30

- Accept null hypothesis - should be no significant difference