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

• 3: 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?

Answer 28

• Number of categories - 1

Question 29

What do the null hypotheses look like?

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?

Answer 30

• Accept null hypothesis - should be no significant difference