TOPIC 7A

Question 1

Give the definition of a gene.

Answer 1

A sequence of bases on a DNA molecule that codes for a protein (polypeptide) which results in a characteristic, e.g. a gene for eye colour.

Question 2

Give the definition of an allele.

Answer 2

A different version of a gene.

There can be many different alleles for a single gene, but most plants and animals, including humans, only carry two alleles of each gene, one from each parent.

The order of bases in each allele is slightly different - they code for different versions of the same characteristic.

They’re represented using letters e.g. the allele for brown eyes (B) and the allele for blue eyes (b)

Question 3

Give the definition of a genotype.

Answer 3

The genetic constitution of an organism - the alleles an organism has e.g. BB, Bb, or bb for eye colour

Question 4

Give the definition of a phenotype.

Answer 4

The expression of the genetic constitution and its interaction with the environment - an organism’s characteristics e.g. brown eyes

Question 5

Give the definition of dominant in terms of alleles.

Answer 5

An allele who’s characteristic appears in the phenotype even when there’s only one copy.

Dominant alleles are shown by a capital letter.

The allele for brown eyes (B) is dominant - If a person’s genotype is Bb or BB, they will have brown eyes.

Question 6

Give the definition of recessive in terms of alleles.

Answer 6

An allele which characteristic only appears in the phenotype if two types are present.

Recessive alleles are shown with a lower case letter.

The for blue eyes (b) is recessive. - If a persons genotype is bb they will have blue eyes.

Question 7

Give the definition of codominant.

Answer 7

Alleles that are both expressed in the phenotype - none of them are recessive, e.g. the alleles for Haemoglobin.

Question 8

Give the definition of a locus.

Answer 8

The fixed position of a gene on a chromosome.

Alleles of a gene are at the same locus on each chromosome in a pair.

Question 9

Give the definition of homozygote.

Answer 9

An organism that carries two copies of the same allele.

e.g. BB or bb

Question 10

Give the definition of a heterozygote.

Answer 10

An organism that carries two different alleles.

e.g. Bb

Question 11

Give the definition of a carrier.

Answer 11

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

Question 12

Are humans diploid or haploid organisms?

Answer 12

Humans are diploid organisms (we have 2 sets of chromosomes) so we have 2 alleles for each gene.

Question 13

Read up on genetic diagrams at the bottom of page 156 and 157 in the revision guide.

Answer 13

Understand?

Question 14

Read top of page 158 in revision guide

Answer 14

Understand?

Question 15

Define the term ‘phenotypic ratio’

Answer 15

The phenotypic ratio is the ratio of different phenotypes in offspring.

Question 16

Look at the table on page 158 in the revision guide.

Able to make notes? not sure, make them if so.

Answer 16

Understand?

Question 17

Describe what it means for an allele to be sex-linked.

Answer 17

The genetic information for gender is carried on two sex chromosomes.

In mammals, females have two X chromosomes (XX) and makes have one X and. one Y chromosome (XY).

The characteristic is said to be sex-linked when the allele that codes for it is located on a sex chromosome.

Question 18

Why are genetic disorders that are sex-linked more common in males than in females?

Answer 18

Males only have one X chromosome, they often have one allele for sex-linked genes. So because they only have one copy, they express the characteristic of this allele even if its recessive.

This makes males more likely than females to show the recessive phenotypes for genes that are sex linked.

Question 19

Look at the pink boxed example on page 159 in the revision guide.

Answer 19

understand?

Question 20

What is an autosome?

Answer 20

Autosome is the fancy name for any chromosome that isn’t a sex chromosome.

Question 21

What are autosomal genes?

Answer 21

Autosomal genes are the genes located on the autosomes.

Question 22

What does it mean when we say some autosomal genes are linked?

Answer 22

Genes on the same autosome are said to be linked-

Because their on the same autosome they’ll stay together during independent segregation of chromosomes in meiosis I, and their alleles will be passed on to the offspring together.

The only reason this won’t happen is if crossing over splits them up first.

The closer 2 genes are on the autosome, the more closely they are said to be linked. This is because crossing over is less likely to split them up.

If two genes are autosomally linked, you won’t get the phenotypic ratio you expect in the offspring of a cross.

(see last two points on page 159 in the revision guide for more example n info)

Question 23

Read the example on page 160 in the revision guide.

Answer 23

Understand?

Question 24

Define epistasis.

Answer 24

Many different genes can control the same characteristic - they interact to form the phenotype.

This can be because the allele of one gene masks (blocks) the expression of the alleles of other genes - this is called epistasis.

(see page 160 in the revision guide for 2 examples)

Question 25

Read the 2 blue boxes on page 160 in the revision guide.

Answer 25

Understand?

Question 26

In the chi-squared test, what is the expected result?

Answer 26

The theory is used to predict a result.

Question 27

In the chi-squared test, what is the observed result?

Answer 27

Thee experiment is carried out and the actual result is recorded.

Question 28

Give a general null hypothesis for a chi-squared test.

Answer 28

There’s no significant difference between the observed and expected results, any difference will be due to chance.

Question 29

Read the green box on page 162 in the revision guide.

Answer 29

Understand?

Question 30

Read the bottom of page 163 in the revision guide.

Answer 30

Understand?

Question 31

If your chi-squared value is larger than or equal to the critical value:

Are the results significant?

Answer 31

There is a significant difference between the observed and expected results, the results are not due to chance, we reject the null hypothesis.

Question 32

If your chi-squared value is smaller than the critical value:

Are the results significant?

Answer 32

There is no significant difference between the observed and expected results, any difference is due to chance, we accept the null hypothesis.

Question 33

Read the green box on page 163 in the revision guide.

Answer 33

Understand?