Genetics and Inheritance

Question 1

What is a gene?

Answer 1

A gene is a section of DNA, that codes for a specific protein/polypeptide.

Question 2

What is a genotype?

Answer 2

The genetic constitution of an organism. All the alleles that an organism possesses.

Question 3

What is a gene pool?

Answer 3

All the alleles within an interbreeding population (at a specific time).

Question 4

What is a population?

Answer 4

All the individuals of the same species that occupy the same area/habitat at the same time.

Question 5

What is a phenotype?

Answer 5

The expression of an organism’s genetic constitution and its interaction with the environment.

Question 6

What is an allele?

Answer 6

Different forms of the same gene.

Question 7

What is a locus?

Answer 7

Different alleles for the same gene are found at the same position on the homologous chromosome; this position is referred to as the locus of that gene.

Question 8

What is meant by homozygous?

Answer 8

If the two copies of a gene are the same allele, the organism is said to be homozygous.

Question 9

What is meant by heterozygous?

Answer 9

The two copies of a gene are different alleles.

Question 10

What is a dominant allele?

Answer 10

If an allele is dominant, it will always be expressed in the phenotype regardless of whether an organism’s genotype is homozygous or heterozygous

Question 11

What is a recessive allele?

Answer 11

If an allele is recessive, it will only be expressed in the phenotype if the genotype is homozygous.

Question 12

What are codominant alleles?

Answer 12

Equally expressed within the phenotype.

Question 13

What is meant by diploid?

Answer 13

When a cell has two copies of a gene. These may be the same (homozygous) or different (heterozygous).

Question 14

What is meant by haploid?

Answer 14

When cells only have one copy of a gene, eg. gametes.

Question 15

What do genetic crosses do?

Answer 15

Show the way in which alleles are passed from one generation to the next.

Question 16

State three causes of genetic variation.

Answer 16

1. Mutation
2. Crossing over
3. Independent segregation (of homologous chromosomes)
4. Random fusion of gametes / fertilisation

Question 17

What is meant by a genome?

Answer 17

• (All) the DNA in a cell/organism

- The total number of DNA bases in a cell/organism

Question 18

How do multiple alleles of a gene arise?

Answer 18

• Mutations

- Which are different/at different position in the gene

Question 19

In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios. Suggest two reasons why.

Answer 19

1. Small sample size
2. Fusion/fertilisation of gametes is random
3. Linked genes, sex linkage, crossing over
4. Epistasis
5. Lethal genotypes

Question 20

What does Hardy Weinberg’s equation predict?

Answer 20

• The frequency/proportion of alleles (of a particular gene)
• Will stay constant from one generation to the next
• Providing no mutation/no selection/large population/population genetically isolated/random mating/no migration

Question 21

Define gene linkage.

Answer 21

(Genes/loci) on the same chromosome

Question 22

Define epistasis

Answer 22

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

Question 23

Describe why observed phenotypes don’t match expected values.

Answer 23

• Fertilisation is random OR fusion of gametes is random
• Small population/sample
• Selection advantage/lethal alleles

Question 24

Rules for dominant alleles.

Answer 24

• Affected offspring must have at least one affected parent
• Unaffected parents only have unaffected offspring
• If both parents are affected and have an unaffected offspring, both parents must be heterozygous

Question 25

Rules for recessive alleles.

Answer 25

-Unaffected parents can have an affected offspring (if they are both heterozygous)

Question 26

Male offspring are more likely than females to show recessive sex-linked characteristics. Explain why.

Answer 26

• (Recessive) allele is always expressed in males / males have one (recessive) allele
• Females need two recessive alleles

Question 27

Expected offspring phenotype ratios from heterozygous parents for monohybrid inheritance.

Answer 27

Dominant:recessive
3:1

Question 28

Expected offspring phenotype ratios from heterozygous parents for dihybrid inheritance.

Answer 28

Dominant : recessive

9:3:3:1

Question 29

Expected offspring phenotype ratios from heterozygous parents for epistasis.

Answer 29

Dominant : recesive

9:4:3 or 15:1 or 9:7

Question 30

Expected offspring phenotype ratios from heterozygous parents for autosomal linkage.

Answer 30

Dominant : recessive

3:1 (no crossing over) (no othee pattern other than 4 phenotypes with recombination of alleles)

Question 31

Explain how a single base substitution causes a change in the structure of a polypeptide.

Answer 31

• Change in (sequence of) amino acid(s)/primary structure
• Change in hydrogen/ionic/disulfide bonds
• Aletrs tertiary/3° structure