3.7.1 Inheritance (Genetics , population , evolution and ecosystems 3.7)

Question 1

What is meant by the term genotype

Answer 1

Genetic constitution of an organism

Question 2

What is meant by the term phenotype

Answer 2

The expression of this genetic constitution and its interaction with the environment

Question 3

What are alleles and how do they arise

Answer 3

Variations of a particular gene - arise by mutation

Question 4

How many alleles of a gene can be found in a diploid organism

Answer 4

• 2 as dipole is organisms have 2 sets of chromosomes
  • but there may be many alleles of a single gene in a population

Question 5

Describe the e different types of alleles

Answer 5

Dominant alleles : Always expressed
Recessive alleles : Only expressed when 2 copies present ( homozygous recessive )
Codominant alleles : Both alleles expressed

Question 6

What is meant by the term homozygous and heterozygous

Answer 6

Homozygous:
Alleles at a specific locus are the same
Heterozygous:
Alleles at a specific locus are different

Question 7

What do monohybrid and dihybrid crosses show

Answer 7

• monohybrid cross = inheritance of one phenotypic characteristics coded for by a single gene
• Dihybrid cross = inheritance of two phenotypic characteristics coded for by two different genes

Question 8

Explain the evidence from a pedigree diagram which would show that the allele for [ named phenotype ] is dominant

Answer 8

• [Named phenotype] parents [n&n] have child [n] without [named phenotype]
• So both parents [n&n] must be heterozygous/ carriers of recessive allele
- if it were recessive , all offspring would have [ named phenotype]

Question 9

Explain the evidence from a pedigree diagram which would show that the allele for [named phenotype ] is recessive

Answer 9

• Parents [n&n] without [named phenotype] have child [n] with [named phenotype]
• so both parents [n&n] must be heterozygous / carriers of recessive allele

Question 10

What is a sex linked gene

Answer 10

A gene with a locus on a sex chromosome

Question 11

Explain why males are more likely to express a recessive X- linked allele

Answer 11

• Females (XX) have 2 alleles => only express recessive allele if homozygous recessive / can be carriers
• Males (XY) have 1 allele -> recessive allele always expressed

Question 12

Explain the evidence from a pedigree diagram which would show that the allele for [named phenotype ] on the X chromosome is recessive

Answer 12

• mother [n] without [named phenotype] has a child [n] with [named phenotype]
• so mother [n] must be heterozygous / carrier of recessive allele

Question 13

Explain the evidence from a pedigree diagram which would suggest that [named recessive phenotype] is caused by a gene on the X chromosome

Answer 13

only males tend to have [named recessive phenotype]

Question 14

Explain the evidence from a pedigree diagram which would show that the gene for [named phenotype] is not on the X chromosome

Answer 14

• [named phenotype] father [n] has daughter [n] without [named phenotype]
• father [n] would pass on allele for [named phenotype] on X chromosome so daughter [n] would have [named phenotype]

Question 15

explain how autosomal linkage affects inheritance of alleles

Answer 15

• two genes located on same autosome ( non - sex chromosome )
• so allele on same chromosome inherited together
  • stay together during independent segregation of homologous chromosomes during meiosis
• But crossing over between homologous chromosomes can create new combinations of alleles
• if the genes are closer together on an autosomes , they’re less likely to be split by crossing over

Question 16

What is epistasis

Answer 16

interaction of non linked genes where one masks / suppresses the expression of the other

Question 17

describe when a chi-square test (X^2) can be used

Answer 17

• when determining if observed results are significantly different from expected results
• data is categorical

Question 18

suggest why in genetic crosses , the observed phenotypic ratios obtained in the offspring are not often the same as the expected ratios

Answer 18

• fertilisation is random
• autosomal linkage / epistasis / sex linkage
• small sample size - not representative of the whole population
• some genotypes may be lethal

Question 19

describe how a chi-square value can be calculated

Answer 19

x^2 = sum of (O- E) ^2 / E

O = frequencies observed
E = frequencies expected

Question 20

Describe how a chi-squared value can be analysed

Answer 20

1) number of degrees of freedom = number of categories - 1
2) determine critical value at p=0.05% from a. table
3) if x^2 value is [greater / less] than critical value at p< 0.05
• difference [is/is not] significant so [reject / accept ] null hypothesis
• so there is [ less / more] than 5% probability that the difference is due to chance