Genetics

Question 1

Gene?

Answer 1

A sequence of bases on a DNA molecule that codes for a protein which results in a characteristic.

Question 2

Alleles?

Answer 2

More than one version of the same gene. There can be many different alleles of a single gene but most plant and animals only carry two alleles of each gene, one from each parent. The order of the bases in each allele is slightly different and are represented using letters.

Question 3

Locus?

Answer 3

The fixed position on a chromosome where a particular allele is found.

Question 4

What is meant by diploid organisms?

Answer 4

Humans are diploid organisms, this means we have two copies of each chromosome, one from each parent.

Question 5

Genotype?

Answer 5

An organisms genetic constitution, the different alleles an organism has. For example, one person may have the genotype BB for eye colour and another Bb for it.

Question 6

Phenotype?

Answer 6

The expression of the genetic constitution and its interactions with the environment. For example, one person may have brown eyes and another may have blue eyes.

Question 7

Dominant alleles?

Answer 7

A dominant allele is always expressed in the phenotype, even when there’s only one copy of it. They are shown by a capital letter.

Question 8

Recessive alleles?

Answer 8

A recessive allele are those with characteristics that only appear in the phenotype if two copies are present. They are shown by a lower case letter.

Question 9

Co-dominant alleles?

Answer 9

Some alleles are both expressed in the phenotype because neither one is recessive.

Question 10

Homozygous?

Answer 10

When an organism carries two copies of the same allele at the same locus.

Question 11

Heterozygous?

Answer 11

When an organism carries two different alleles at the same locus.

Question 12

Genetic diagrams?

Answer 12

Genetic diagrams can be used to predict the genotypes and phenotypes of the offspring produced if two parents are crossed.

Question 13

What happens when haploid gametes of two parents fuse together?

Answer 13

The alleles they contain form the genotype of the diploid offspring that is produced.

Question 14

Monohybrid inheritance?

Answer 14

The inheritance of a characteristic controlled by a single gene.

Question 15

Monohybrid crosses?

Answer 15

The likelihood of the different alleles of that gene being inherited by the offspring of certain parents.

Question 16

Phenotypic ratios?

Answer 16

The ratio of different phenotypes in the offspring. Genetic diagrams allow you to predict the phenotypic ratios in F1 and F2 offspring.

Question 17

Punnet square?

Answer 17

Another way of showing a genetic diagram.

Question 18

Monohybrid inheritance of codominant alleles?

Answer 18

Occasionally, alleles show co-dominance, both alleles are expressed in the phenotype and neither one is recessive.

Question 19

What is an example of monohybrid inheritance of codominant alleles?

Answer 19

Sickle cell anaemia.

Question 20

Multiple allele crosses?

Answer 20

Inheritance is more complicated when there are more than two alleles of the same gene.

Question 21

Dihybrid crosses?

Answer 21

Dihybrid crosses can be used to show the likelihood of offspring inheriting certain combinations of the two characteristics from particular parents.

Question 22

Dihybrid inheritance?

Answer 22

The inheritance of two characteristics, which are controlled by different genes. Each of two genes will have different alleles.

Question 23

Sex-linked characteristics?

Answer 23

When the allele that codes for the characteristic is located on a sex chromosome (X or Y)

Question 24

Sex-linked genetic disorders?

Answer 24

Colour blindness and haemophilia. The faulty alleles for both of these disorders are carried on the X chromosome and are so called X-linked disorders. Y-linked disorders do exist but are less common.

Question 25

Males and sex-linked genetic?

Answer 25

As males only have one X chromosome and one Y chromosome they only need one version of the sex-linked allele to show the phenotype even if it is recessive.

Question 26

Autosomal genes?

Answer 26

Any chromosome that isn’t a sex chromosome.

Question 27

Autosomal linked genes?

Answer 27

They are said to be linked because they will stay together during the independent segregation of chromosomes in meiosis I and their alleles will be passed on to the offspring together. The only reason this wont happen is if crossing over splits them up first. The closer together two 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 wont get the phenotypic ratio you’d expect.

Question 28

Epistasis?

Answer 28

When an allele of one gene masks the expression of the alleles of other genes.

Question 29

Recessive epistasis?

Answer 29

If the epistasis allele is recessive then two copies of it will mask the expression of the other gene.

Question 30

Dominant epistasis?

Answer 30

If the epistasis allele is dominant, then having at least one copy of it will mask the expression of the other gene.