7A: Inheritance

Question 1

Define gene

Answer 1

A sequence of bases on a DNA molecule that codes fr a protein which results in a characteristic e.g eye colour

Question 2

Define allele

Answer 2

A different version of a gene.

• There can be many different alleles on a single gene, but in diploid organisms (e.g humans) only 2 are carried.
• The order of bases on each allele is slightly different –> they code for different versions of the same characteristic

Question 3

Define genotype

Answer 3

The genetic constitution of an organism - the alleles it has

Question 4

Define phenotype

Answer 4

The expression of the genetic constitution and its interaction with the environment

Question 5

Define dominant

Answer 5

An allele whose characteristic appears in the phenotype even when there’s only one copy. Shown by a capital letter

Question 6

Define recessive

Answer 6

An allele whose characteristic only appears in the phenotype if 2 copies are present. Shown by a lower case letter

Question 7

Define codominant

Answer 7

When alleles are both expressed in the phenotype - neither one is recessive

Question 8

Define locus

Answer 8

The fixed position of a gene on a chromosome. Alleles of a gene are found at the same locus on each chromosome in a pair

Question 9

Define homozygote

Answer 9

An organism that carries 2 copies of the same allele, e.g BB or bb

Question 10

Define heterozygote

Answer 10

An organism that carries two different alleles, e.g Bb

Question 11

Define carrier

Answer 11

A person carrying an allele which is not expressed in the phenotype but can be passed to the offspring

Question 12

What are diploid organisms?

Answer 12

They carry 2 sets of chromosomes

Question 13

Define gametes

Answer 13

Sex cells

Question 14

Define monohybrid inheritance

Answer 14

The inheritance of a characteristic controlled by a single gene

Question 15

In the F1 generation of a monohybrid cross, what ratio of phenotypes would you expect to see in the offspring?

Answer 15

In F1 of a monohybrid cross, a homozygous dominant organism is bred with a homozygous recessive. This will produce offspring that are all heterozygous dominant

Question 16

In the F2 generation of a monohybrid cross, what ratio of phenotypes would you expect to see in the offspring?

Answer 16

2 heterozygotes (F1) are crossed. In F2, you’d expect a ratio of 3:1 dominant to recessive phenotypes

Question 17

Define dihybrid inheritance

Answer 17

When 2 different genes, on different chromosomes are inherited together

Question 18

In the F2 generation of a dihybrid cross, what would be the expected phenotypic ratio? (providing no linkage/ epistasis is involved)

Answer 18

The expected ratio would be 9 : 3 : 3 : 1

Both dominant : Dominant 1st recessive 2nd : Recessive 1st Dominant 2nd: both recessive

Question 19

In the F1 generation of a dihybrid cross, what ratio of phenotypes would you expect to see in the offspring?

Answer 19

In F1 of a dihybrid cross, a homozygous dominant organism is bred with a homozygous recessive. This will produce offspring that are all heterozygous dominant

Question 20

What is codominance?

Answer 20

When both alleles are expressed in the phenotype, neither one is recessive

Question 21

In the F1 generation of a codominant cross, what ratio of phenotypes would you expect to see in the offspring?

Answer 21

In F1 of a codominant cross, a homozygous dominant organism is bred with a homozygous recessive. This will produce offspring that are all heterozygous dominant

Question 22

In the F2 generation of a codominant cross, what ratio of phenotypes would you expect to see in the offspring?

Answer 22

The expected ratio would be 1 : 2 :1

homozygous for 1 allele : heterozygous for the other allele

Question 23

What is an example of a gene having multiple alleles?

Answer 23

In humans, there are 3 alleles for blood group (ABO)

Question 24

What are the alleles for blood group in humans?

Answer 24

Io = Group O
IA = Group A
IB = Group B
IB IA = Group AB

Question 25

Describe the alleles for blood group

Answer 25

Io = recessive

IA and IB are codominant

Question 26

If someone has the alleles ‘IA IB’ what blood group will they be?

Answer 26

Group AB

Question 27

Describe the offspring of a cross between a heterozygote of blood group A and a heterozygote of blood group B

Answer 27

Any offspring could have 1 of the 4 blood types. 
IA IB = AB 
IA Io = A 
IB Io = B 
Io Io = O

Question 28

What would a person’s genotype be if they were Heterozygous blood group A?

Answer 28

IA Io

Question 29

What chromosomes do females have?

Answer 29

XX

Question 30

What chromosomes do men have?

Answer 30

XY

Question 31

Describe what ‘sex linkage’ means

Answer 31

When an allele is located on a sex chromosome

Question 32

Give an example of sex linkage, what chromosome is this disorder carried on?

Answer 32

Colour blindness - carried on the X chromosome

Question 33

What is meant by autosomal gene linkage?

Answer 33

Genes on the same autosome are said to be linked - because they’ll stay together during independent segregation in meiosis 1 and their alleles will be passed on to their offspring together

Question 34

What process may stop autosomally linked genes from being passed on together?

Answer 34

Crossing over

Question 35

The what effect does the closeness of two genes on the autosome have on their chances of being inherited together?

Answer 35

The closer they are together the more closely they are said to be linked - because crossing over is less likely to split them up

Question 36

What is an autosome?

Answer 36

Any chromosome that is NOT a sex chromosome

Question 37

Describe epistasis

Answer 37

When one gene masks the expression of another gene

Question 38

If two genes are autosomally linked, you won’t get the expected phenotypic ratio. What does this allow you to do?

Answer 38

You can use the predicted phenotypic ratio to identify autosomal linkage

Question 39

What is the chi squared test used for?

Answer 39

To see if the results of an experiment support a theory.

Question 40

What is a null hypothesis?

Answer 40

The null hypothesis is always that there is no significant difference between the observed and expected results (if there is a small difference that it is due to chance, not your theory being wrong)

Question 41

In a chi-squared test, what do these values stand for?
O =
E =
Σ =

Answer 41

O = Observed results
E = Expected results
Σ = Sum of...

Question 42

What is the critical value in a chi-squared test?

Answer 42

The value that you compare your calculated (chi-squared) value to.

Question 43

In a chi-squared test, in what circumstance do you reject the null hypothesis?

Answer 43

If your calculated value is larger than or equal to the critical value (this means there IS a significant difference between O and E)

Question 44

In a chi-squared test, in what circumstance do you accept the null hypothesis?

Answer 44

If your calculated value is smaller than the critical value (this means there is NOT a significant difference between O and E)

Question 45

How do you calculate degrees of freedom?

Answer 45

No. of classes - 1