Inheritance

Question 1

Zygote

Answer 1

Fertilised egg cell

Question 2

Allele

Answer 2

Alternative form of a gene

Question 3

Dominant gene

Answer 3

Always expressed in phenotype

Question 4

Recessive gene

Answer 4

Only expressed in phenotype when homozygous

Question 5

Homozygous

Answer 5

Pair of chromosomes that have two of the same alleles of a gene

Question 6

Heterozygous

Answer 6

Pair of chromosomes that have two different alleles of a gene

Question 7

Genotype

Answer 7

Combination of alleles for a particular gene

Question 8

Phenotype

Answer 8

Expression of characteristic due to genotype AND environment

Question 9

Monohybrid inheritance

Answer 9

Inheritance of a single phenotype affected by one gene

Question 10

Acronym for Genetic Diagram

Answer 10

First
Inside
Outside
Last

Question 11

Genetic Diagram Points (5)

Answer 11

(Parent phenotype)
Parent genotype 
Gametes
Offspring genotype (in table) 
Offspring phenotype with proportions (for monohybrid match genotype to phenotypes but with dihybrid give phenotypes in ratio)

Question 12

Test cross

Answer 12

• Determine if individual with dominant characteristic is homozygous or heterozygous
• Crossbred with homozygous recessive individual (give genotype)
• All offspring dominant = homozygous
• Some offspring recessive = heterozygous

Question 13

How to prove gene is dominant

Answer 13

Affected parents produce unaffected offspring

Question 14

How to prove gene is recessive

Answer 14

Unaffected parents produce affected offspring

Question 15

Codominant gene

Answer 15

Both alleles are equally dominant so both are expressed in phenotype

Question 16

Multiple Alleles

Answer 16

More than two alternative forms of a gene (alleles) that can occupy the same locus, of which only two can be present in a single organism’s homologous chromosomes

E.g. ABO system of blood groups is controlled by three alleles, only two of which are present in an individual

Question 17

Sex-linked gene

Answer 17

A gene found only on sex chromosomes, i.e. X chromosomes in humans

Question 18

Differences between X and Y chromosomes

Answer 18

• X larger compared to Y

- X contains more genes

Question 19

Suggest why sex-linked diseases are more common in men compared to women

Answer 19

• men only have one X chromosome
• Y chromosome does not have an equivalent non homologous portion as X
• recessive alleles on non homologous portion of X are more frequently expressed

Question 20

Dihybrid Inheritance

Answer 20

Inheritance of two phenotypes determined by different genes located on different chromosomes

Question 21

Dihybrid Cross

Answer 21

Mating experiment between two organisms both heterozygous for different traits

Gives theoretical ratio 9:3:3:1

Question 22

What does dihybird cross prove

Answer 22

• 9:3:3:1 ratio
• pairs of alleles for a trait are sorted independently from one another from generation to generation
• independent assortment

Question 23

Suggest why there are four possible gametes in a dihybrid cross

Answer 23

• genes for both traits on separate chromosomes

- independent segregation occurs during meiosis where chromosomes arrange themselves randomly along equator

Question 24

Autosomal Linkage

Answer 24

Two or more genes carried on the same autosome (non-sex chromosome) so are linked

Question 25

What impact does cross over have on autosomal linkage

Answer 25

Cross over creates new combinations of alleles so there are four possible gametes as oppose to two

Question 26

What determines if two genes are likely to be separated during cross over

Answer 26

Two alleles are less likely to be separated during cross over if they are closer together on chromosome

Question 27

Epistasis

Answer 27

When one gene prevents the expression of another gene

Question 28

Dominant Epistasis

Answer 28

Epistatic gene requires at least one dominant allele to prevent expression of other gene

Question 29

Recessive Epistasis

Answer 29

Epistatic gene must be homozygous recessive to prevent expression of other gene

Question 30

Phenotypic ratios when breeding parents that are both heterozygous for two genes

Answer 30

9: 3:3:1 (independent genes/no epistasis) 9: 3:4 (recessive epistasis) 12: 3:1 or 13:3 (dominant epistasis)

Question 31

Explain why observed ratios are often not the same as expected ratios in a population

Answer 31

- small sample size - random fusion of gametes - lethal alleles / selective disadvantage

Question 32

How do multiple alleles of a gene arise

Answer 32

mutations

Question 33

Suggest what dashes represent in a genotype

Answer 33

Either dominant or recessive allele

Question 34

Suggest what assumptions are made when using Hardy-Weinberg equation

Answer 34

- no selection - random mating - large population / gene pool - no mutations - population is isolated - (generations do not overlap) - (no immigration or emigration)

Question 35

Suggest why there might be a high incidence of a genetic disease in a population

Answer 35

- common ancestor - genetic isolation / small gene pool - in breeding - high probability of mating with individual carrying disease causing allele

Question 36

Suggest how results of a genetic cross demonstrate independent assortment is involved in production of gametes

Answer 36

- new combinations of alleles | - in offspring but not parents

Question 37

Explain how evidence from a pedigree diagram shows a condition is recessive

Answer 37

- unaffected parents have a child with condition | - both parents must be carriers

Question 38

Suggest why sex linked genes are found only on the X chromosome

Answer 38

- X chromosome is larger | - Y lacks equivalent non homologous section where sex-linked genes are found

Question 39

Suggest why number of people with a fatal genetic disease may actually be lower than predicted by the Hardy-Weinberg equation

Answer 39

Some of those with condition die

Question 40

Suggest what the Hardy-Weinberg principle predicts

Answer 40

The proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next

Question 41

Describe how to determine if a trait in an organism is genetic or environmental

Answer 41

- breed in a lab - observe / measure trait - if trait is present then must be genetic and not environmental

Question 42

Explain how you can tell genes are linked

Answer 42

- certain phenotypes are more common when you would expect equal numbers - alleles controlling more common phenotype are linked - gametes mainly produce linked combination of alleles - cross over rarely produces other combinations of alleles

Question 43

Suggest reasons why ratio of offspring might not be the same as predicted

Answer 43

``` - epistasis OR - autosomal linkage - no independent segregation - linked alleles inherited in same gamete UNLESS cross over occurs ```