Topic 3 - Inheritance

Question 1

What is a chromosome?

Answer 1

A long, coiled molecule of DNA that carries genetic information in the form of
genes

Question 2

Define gene

Answer 2

A section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein

Question 3

What are alleles?

Answer 3

Different versions of the same gene

Question 4

Define genotype

Answer 4

An organism’s genetic composition,

describes all alleles

Question 5

Define phenotype

Answer 5

An organism’s observable characteristics due
to interactions of the genotype and environment
(which can modify the phenotype)

Question 6

Define homozygous

Answer 6

Having two identical alleles of a gene e.g. FF or ff

Question 7

Define heterozygous

Answer 7

Having two different alleles of a gene e.g. Ff

Question 8

What is a dominant allele?

Answer 8

Describes an allele that is always expressed

Represented with a capital letter e.g. F

Question 9

What is a recessive allele?

Answer 9

An allele that is only expressed in the
absence of a dominant allele
Represented with a small letter e.g. f

Question 10

What is monohybrid inheritance?

Answer 10

The inheritance of a single gene

Question 11

Imagine if … parents who are both heterozygous for sickle cell anaemia (Aa) have a child. Draw a genetic diagram to illustrate this single gene inheritance.

Answer 11

AA, Aa, Aa and aa

Question 12

A female who is homozygous recessive for cystic fibrosis (ff) has a child with a heterozygous male (Ff). Draw a punnett square to illustrate this single gene inheritance

Answer 12

Ff, Ff, ff and ff

Question 13

PKU is a recessive condition. Two heterozygous parents (Pp) have offspring. Predict the proportion of offspring that will have PKU.

Answer 13

PP, Pp, Pp and pp

Question 14

What is the problem with single gene crosses?

Answer 14

Most characteristics are controlled by multiple alleles rather than just one

Question 15

What are sex chromosomes?

Answer 15

A pair of chromosomes that determine sex:
● Males have an X and a Y chromosome
● Females have two X chromosomes

Question 16

Why does the inheritance of a Y chromosome mean that an embryo develops into a male?

Answer 16

Testes development in an embryo is stimulated by a gene present on the Y
chromosome

Question 17

A couple have a child. Using a punnett square, determine the probability of having offspring that is female.

Answer 17

X X
Y XY XY
X XX XX

Question 18

Other than using a punnett square, how else can monohybrid inheritance be
represented?

Answer 18

Using a family pedigree

Question 19

What is a sex-linked characteristic?

Answer 19

A characteristic that is coded for by an allele found on a sex chromosome.

Question 20

Why are the majority of genes found on the X chromosome rather than the Y chromosome?

Answer 20

The X chromosome is bigger than the Y

chromosome so more genes are carried on it.

Question 21

Why are men more likely to show the phenotype for a recessive sex-linked trait
than women?

Answer 21

● Many genes are found on the X chromosome that have no
counterpart on the Y chromosome
● Women (XX) have two alleles for each sex-linked gene whereas men
(XY) often only have one allele ∴ only one recessive allele is required
to produce the recessive phenotype in males

Question 22

Haemophilia is a recessive X-linked condition. A carrier female and a normal
male have a son. What is the probability of the child having haemophilia?

Answer 22

50%

Question 23

Give an example of a characteristic that is determined by more than one allele

Answer 23

Blood group is determined by three

alleles: I^A, I^B, I^O

Question 24

Name the four different blood groups

Answer 24

A, B, AB, O

Question 25

What are codominant alleles?

Answer 25

Alleles that equally contribute to an organism’s phenotype. They are expressed to an equal extent.

Question 26

Describe codominance in blood groups

Answer 26

● I^A and I^B are codominant
● I^A I^B
gives the blood group AB

Question 27

Why does I^A I^O give blood group A?

Answer 27

● I^O is recessive to I^A
● I^A is dominant and is expressed giving
blood group A

Question 28

What are the possible genotypes for blood group B?

Answer 28

I^B I^O and I^B I^B

Question 29

What is the genotype for blood group O?

Answer 29

I^O I^O