Session 7: Genotype, phenotype & inheritance

Question 1

Which type of inheritance does Cystic Fibrosis show?

Answer 1

Autosomal dominant

Question 2

What does homozygous mean?

Answer 2

2 alleles of a gene are the same

Question 3

What does heterozygous mean?

Answer 3

2 alleles of a gene are different

Question 4

What is a gene?

Answer 4

A stretch of DNA - the basic unit of hereditary passed from parent to child

Question 5

How many alleles does each gene have?

Answer 5

2

Question 6

What is an allele?

Answer 6

an alternative version of the same gene as the same position on the chromosome

Question 7

What is phenotype?

Answer 7

the observable characteristics of an individual resulting from interaction of its genotype with the environment

Question 8

What is genotype?

Answer 8

the combination of alleles for a specific gene

Question 9

What does hemizygous mean?

Answer 9

there is only one allele (on X chromosome)

Question 10

What does recessive mean?

Answer 10

non-dominant allele in a heterozygote (it is not expressed, in Aa, small a is recessive but not expressed in presence of dominant allele)

Question 11

What does dominant mean?

Answer 11

dominant allele in a heterozygote determines the phenotype (ex Aa or AA)

Question 12

What is an autosomal disease?

Answer 12

disease gene located on non-sex chromosome

Question 13

What is a sex-linked disease?

Answer 13

gene on X or Y chromosome

Question 14

What is codominance?

Answer 14

2 alleles are both expressed at the same time

Question 15

Describe features of autosomal dominant diseases. Give an example.

Answer 15

• Single copy of gene leads to phenotype being expressed
• One parent must have the trait
• Heterozygotes affected (eg Hh), rarely found in homozygous form (HH)
• 2 heterozygotes have 50% chance of having affected offspring
• Doesn’t skip generations
• Example: Huntington’s disease

Question 16

Describe features of autosomal dominant diseases. Give an example.

Answer 16

• Single copy of gene leads to phenotype being expressed
• One parent must have the trait
• Heterozygotes affected (eg Hh), rarely found in homozygous form (HH)
• 2 heterozygotes have 50% chance of having affected offspring
• Doesn’t skip generations
• Example: Huntington’s disease

Question 17

Describe features of autosomal recessive diseases. Give an example.

Answer 17

• Able to skip generations
• Must have 2 affected chromosomes for disease to present (eg aa)
• Heterozygotes = carriers but unaffected (dominant allele dominates recessive)
• 2 heterozygotes = 25% chance of having affected offspring
• 2 homozygous = only have affected offspring
  Example: Cystic fibrosis

Question 18

What type of inheritance does sickle cell anaemia show?

Answer 18

Autosomal recessive disease

Question 19

Describe features of X-linked recessive diseases.

Answer 19

• Affected gene on X chromosome
• For female to be affected, they must inherit recessive allele from mother AND father (2 copies of X)
• For male to be affected they only need one copy of recessive allele from mother

Question 20

Why do affected males have heterozygous daughters in X-linked recessive inheritance?

Answer 20

They can only pass on 1 X copy of X chromosome and they do not pass on Y chromosome

Question 21

Describe features of X-linked dominant diseases.

Answer 21

• Hemizygous males (Xx,Y) & heterozygous females (XX, Xx) are affected
• Heterozygous females (XX, Xx) = 50% chance of affected offspring
• Affected males cannot give trait to sons = because they only pass on Y chromosome to sons
• Affected males give trait to daughters = pass on single X chromosome that has dominant allele

Question 22

Why do affected males not pass on the dominant x-linked trait to their sons?

Answer 22

Because they only pass on Y chromosome to sons

Question 23

Why do affected males only pass trait to daughters in x-linked dominant diseases?

Answer 23

Because they only pass on Y chromosome to sons & they pass on single X chromosome that has dominant allele

Question 24

Describe features of Y-linked dominant diseases.

Answer 24

• Disease causing allele on Y chromosome = so only expressed in males
• Carriers in all generations

Question 25

What are the letters used to describe affected males in X-linked dominant disease? If D = have disease & d = not disease.

Answer 25

Affected males: XDY
Unaffected males: XdY

Question 26

What are the letters used to describe affected females in X-linked dominant disease? If M = have disease & m = not disease.

Answer 26

Affected females: XMXm or XMXM
Unaffected females: XmXm

Question 27

What are the letters used to describe affected females in X-linked recessive disease? If M = not disease & m = have disease.

Answer 27

Affected: XmXm
Unaffected: XMXm or XMM

Question 28

What is mitochondrial inheritance?

Answer 28

Mitochondria contain a small circular chromosome, DNA (mtDNA) that is only inherited from mother

Question 29

Why is mitochondrial DNA only inherited from mother?

Answer 29

Because mitochondria of egg used to produce all mitochondrial for cells of body

Question 30

What happens if there is a mutation of mitochondrial DNA (mtDNA)?

Answer 30

cause malformation of proteins needed to regenerate large amount of ATP from oxidative phosphorylation needed by cells

Question 31

What does mtDNA encode for?

Answer 31

Encodes proteins for oxidative phosphorylation

Question 32

What is polygenic inheritance?

Answer 32

phenotype influenced by the activity of multiple genes rather than the activity of one single gene (monogenic)

Question 33

Give an example of polygenic inheritance. How is it polygenic?

Answer 33

• Albinism
• More than 1 gene involved in the expression of albinism (2 copies of A gene)
• 2 albino people can have offspring that does not have albinism because there are 2 different copies of the A gene and so 2 more alleles (4 in total - A1, a1 & A2, a2)
• Homozygous recessive alleles must be present for albinism to be observed: a1a1 or a2a2.

Question 34

What is gene linkage?

Answer 34

genes that are close together on the same chromosome are inherited together as they do not show independent assortment in meiosis

Question 35

What happens for genes that are not linked?

Answer 35

Genes on diff chromosomes are not linked => they assort independently during meiosis and daughter cells will inherit them in set ratio 1:1:1:1

Question 36

What are blocks of linkage formed on chromosomes called?

Answer 36

haplotypes

Question 37

What is linkage disequilibrium?

Answer 37

loci on diff chromosomes are inherited together

Question 38

What is the chromosomal locus?

Answer 38

The exact position of a gene on a chromosome