Topic 7A - Genetics

Question 1

Define these key words:

1. Gene
2. Allele
3. Genotype
4. Phenotype

Answer 1

1. Gene = a sequence of DNA bases that codes for a particular protein.
2. Allele = a different version of a gene
3. Genotype =the combination of alleles (Bb)
4. Phenotype =the organisms characteristics.

Question 2

Define these key words:

5. Dominant
6. Recessive
7. Codominant
8. Locus
9. Homozygote
10. Heterozygous
11. Carrier

Answer 2

5. Dominant: allele whose characteristic appears in phenotype even if there’s only copy.
6. Recessive: allele that needs to 2 copies to appear in phenotype
7. Codominant: alleles that are both expressed in phenotype
8. Locus: the fixed position of a gene on a chromosome. Alleles of a gene are found at the same locus on each chromosome in homologous pair.
9. Homozygote: 2 copies of same allele
10. Heterozygote: 2 copies of different allele.
11. Carrier: A person carrying an allele which is not expressed in their phenotype but can be passed on to offspring

Question 3

practise drawing monohybrid crosses

Answer 3

page 168

Question 4

practise drawing Punnett square diagrams

Answer 4

page 169

Question 5

How many alleles are there for each gene in a diploid organism?

Answer 5

2

Question 6

How many alleles are there for each gene in gametes?

Answer 6

1

Question 7

Define monohybrid inheritance

Answer 7

The inheritance of a characteristic controlled by a single gene

Question 8

practise drawing codominance genetic diagrams

Answer 8

page 169

Question 9

practise drawing blood group genetic diagrams

Answer 9

page 169

Question 10

practise drawing dihybrid crosses

Answer 10

page 170

Question 11

Define phenotypic ratio

Answer 11

The ratio of different phenotypes in offspring

Question 12

What do dihybrid crosses show you?

Answer 12

They show you how 2 different genes are inherited at the same time

Question 13

What are the main differences between the Y chromosome and the X chromosome?

Answer 13

The Y chromosome is smaller than the X chromosomes and carries fewer genes

Question 14

Why are males more likely to inherit a sex-linked disorder such as colour blindness?

Answer 14

• As the X chromosome is bigger, it carries more genes and therefore, determines more characteristics.
• Some genes that are carried by the X chromosome have faulty alleles such as the one for colour blindness.
• As females are XX, they need two copies of the faulty allele to show the characteristic, whilst males only need 1 as the Y chromosome doesn’t carry the faulty allele.

Question 15

Practise drawing genetic diagrams for sex-linked disease

Answer 15

Page 171

Question 16

Define:

1. Autosome

2. Autosomal genes

Answer 16

1. Autosome = Any chromosome that isn’t a sex chromosome

2. Autosomal genes = the genes found on the autosome

Question 17

What is important about genes found on the same autosome?

Answer 17

Genes on same autosome are said to be linked as it’s assumed they stay together during independent segregation of chromosomes in meiosis I, and their alleles will be passed on to offspring together. Only time this wont happen is if crossing over splits them up first
- The closer together 2 genes are on autosome, the more closely they are said to be linked. This is because crossing over is less likely to split them up

Question 18

How can autosomally linked genes affect the phenotypic ratio?

Answer 18

The result would most likely be that which you’d expect in a monohybrid cross, not a dihybrid cross, as the 2 autosomally-linked alleles are inherited together. This means a higher proportion of the offspring will have their parents genotype and phenotype.**

Question 19

practise drawing and explaining genetic diagrams that show autosomal linkage

Answer 19

page 172

Question 20

Does only one single gene control a characteristic?

Answer 20

No, many different genes can interact to control the same characteristic

Question 21

What is epistasis?

Answer 21

Epistasis is when an allele from 1 gene blocks the expression of the alleles of other genes (when they interact to control a phenotype)

Question 22

What do epistatic genes affect (in terms of expected phenotype ratios)?

Answer 22

Crosses involving epistatic genes don’t result in the expected phenotypic ratios as there is no chance a characteristic will be expressed if it is blocked by an epistatic gene.

Question 23

Practise questions on how to predict phenotypes of epistatic genes

Answer 23

page 173

Question 24

Practise questions on chi-squared

Answer 24

pages 174-175