7.1 Inheritance

Question 1

what are 3 principles we can derive from Mendel’s work

Answer 1

• heredity factors do not combine, but are passed on intact
• a parent transmits half its heredity factors to its offspring
• the segregation of heredity factors is random

Question 2

what does a gene do

Answer 2

• codes for a polypeptide, which govern a characteristic
• occupies a specific locus on a chromosome
• is the unit of inheritance

Question 3

how do you find out if an individual showing a dominant phenotype is dominant is homozygous or heterozygous

Answer 3

• cross with individual showing recessive phenotype
• if cross gives offspring uniform dominant phenotypes, the individual is homo
• if the cross gives offspring with dominant and recessive phenotypes, the individual is hetero

Question 4

what is co-dominance

Answer 4

when neither allele is recessive e.g. petal colour in snap dragons

Question 5

what will the expected ratio be when crossing heterozygous co-dominant alleles

Answer 5

1:2:1

Question 6

what are multiple alleles

Answer 6

when there are 2 or more versions of a gene e.g. blood groups, A and B are dominant and is recessive

Question 7

what are sex linked characteristics

Answer 7

when the gene is on one of the sex chromosomes (X or Y)
- males are haploid for genes on the X chromosome

Question 8

example of a sex linked characteristic

Answer 8

haemophilia
- on the X chromosome
- haemophilia will only be expressed in a woman if they are homozygous for the recessive allele
- it will always be expressed in men if they have one recessive allele

Question 9

what will the expected ratio be if a female carrier has children with a normal male

Answer 9

1:1:1:1

Question 10

what are hints to look for in a question if they don’t tell you a characteristic is sex-linked

Answer 10

• more common in males
• affected females have affected fathers

Question 11

what is a dihybrid cross

Answer 11

a crossing involving alleles of two genes e.g. seed texture and seed colour

Question 12

what is the expected ratio for a dihybrid cross that are both heterozygous for both genes

Answer 12

9:3:3:1

Question 13

what is linkage

Answer 13

when 2 or more genes that are located on the same chromosome
- SO they will not segregate independently during meiosis

Question 14

how are linked alleles different in a dihybrid cross

Answer 14

only 2 possible combinations will occur
- so the expected ratio will be 1:2:1 when both parents are heterozygous

Question 15

what does linkage reduce

Answer 15

phenotypic variety

Question 16

what is epistasis

Answer 16

the allele of one gene affects or masks expression of another gene in the phenotype

Question 17

when does epistasis occur

Answer 17

when phenotypes are polygenic (controlled by more than one gene)

Question 18

what could epistasis look like

Answer 18

• when the first enzyme is non-functional, the second would not be seen
• when genes code for a transcription factor

Question 19

why do we use Chi2

Answer 19

to measure how well the observed population data fits the expected data

Question 20

what is the equation to calculate chi2

Answer 20

Question 21

how to calculate E

Answer 21

total population x phenotype probability

Question 22

how to determine the critical value from the table

Answer 22

p = 0.05
degrees of freedom = number of phenotypes - 1

Question 23

what happens if the calculated value is greater than the critical values table

Answer 23

• the probability that the difference between the observed and expected values was due to chance is less than p = 0.05
• we can reject the null hypothesis
• there is a significant difference between the observed and expected values, and the population does not fit the ratio