Inheritance

Question 1

What is monohybrid inheritance?

Answer 1

Inheritance of a single gene

Question 2

What does pure-breeding mean and how does it come about?

Answer 2

Pure-breeding = homozygous for a particular characteristic.
Organisms with the same phenotype are consistently bred together so all offspring have this phenotype and they are homozygous for the gene.

Question 3

Monohybrid inheritance: F1 generation when 2 pure breeding organisms are crossed - genotypes and phenotypes

Answer 3

Phenotype: all dominant
Genotype: all heterozygous

Question 4

Monohybrid inheritance: F2 generation when F1 heterozygous offspring breeding are crossed - genotypes and phenotypes

Answer 4

Phenotype: 3:1 dominant to recessive
Genotype: 1 homozygous dominant: 2 heterozygous: 1 homozygous recessive

Question 5

Why are the actual outcomes of genetic crosses different to the predicted?

Answer 5

due to statistical error: chance determines which gametes fuse with which.

Question 6

How can you make sure the actual results are close to the theoretical ones when carrying out an crossing experiment?

Answer 6

By using a large sample size

Question 7

What is dihybrid inheritance?

Answer 7

how two characteristics, determined by two different genes located on different chromosomes are inherited.

Question 8

General dihybrid cross F1
Parents: XXYY and xxyy
Offspring: ?

Answer 8

all XxYy

all have phenotype of dominant for both genes

Question 9

Why is the allele for one gene independent of the other in dihybrid inheritance?

Answer 9

As the genes are found on different chromosomes, and chromosomes arrange themselves at random during meiosis and independent segregation occurs.
Fertilisation is also random, so any of the 4 types of gametes can combine.

Question 10

Types of gamete for an organism with genotype XxYy

Answer 10

XY, Xy, xY, xy

Question 11

Dihybrid inheritance: theoretical ratio of phenotypes for the F2 generation - cross between 2 XxYy individuals.

Answer 11

9:3:3:1

Question 12

Mendel’s law of independent assortment

Answer 12

each member of a pair of alleles may combine randomly with either of another pair.

Question 13

Codominance

Answer 13

where 2 different alleles of a single gene trait are expressed in the phenotype - instead of one allele being dominant and the other recessive.

Question 14

Xx

which letter represents the dominant allele and which the recessive

Answer 14

X = dominant
x = recessive

Question 15

How are codominant alleles represented?

Answer 15

Different letters are used to represent the alleles, as superscripts to a letter which represents the gene.

Question 16

Why can’t upper and lowercase letters be used to represent codominant genes?

Answer 16

As this would imply that one gene is dominant.

Question 17

Linked genes

Answer 17

genes that are inherited together, i.e. on the same chromosome, so if one gene is inherited, the linked one is too.

Question 18

Unlinked genes

Answer 18

genes that are inherited separately - e.g. on different chromosomes, due to independent segregation.

Question 19

Why can 4 different combinations of alleles in gametes arise with dihybrid inheritance?

Answer 19

Meiosis separates homologous chromosomes and alleles.

Independent segregation means you can get any combination of the 2 alleles an a gamete.

Question 20

Multiple alleles

Answer 20

A gene may have more than 2 alleles, so more different combinations and phenotypes can arise.
Only 2 alleles can be present at one time as there are only 2 homologous chromosomes.

Question 21

Example of multiple alleles

Answer 21

the human ABO blood system

Question 22

Sex-linkage

Answer 22

inheritance of a gene located on one of the sex chromosomes

Question 23

Autosome

Answer 23

any one of the 22 pairs of chromosomes which are not sex chromosomes.

Question 24

Gene

Answer 24

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

Question 25

Allele

Answer 25

A different version of the same gene. Most organisms have 2 alleles of each gene, one on each homologous chromosome.

Question 26

Genotype

Answer 26

The genetic constitution of an organism - all the alleles it has

Question 27

Phenotype

Answer 27

The expression of an organism's genetic constitution and its interaction with the environment, an organism's observable or biochemical characteristics.

Question 28

Dominant

Answer 28

An allele which is expressed in the phenotype even when only one copy is present. Represented with a capital letter.

Question 29

Recessive

Answer 29

An allele which is only expressed in the phenotype if 2 copies are present.

Question 30

Homozygote

Answer 30

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

Question 31

Heterozygote

Answer 31

An organism that carries 2 different alleles of the same gene, e.g. Bb

Question 32

Carrier

Answer 32

An organism carrying an allele which is not expressed in the phenotype but can be passed on to offspring.

Question 33

Why are males more likely to express recessive phenotypes?

Answer 33

as the X chromosome is much longer than the Y and carries more genes, so most of the X chromosome doesn't have an equivalent homologous portion of the Y chromosome. Males only have 1 X chromosome so often only have 1 allele for sex-linked genes. This allele is always expressed, even if it is recessive.

Question 34

Autosomal linkage

Answer 34

Inheritance of 2+ genes which are carried on the same autosome.

Question 35

Why are females less likely to be colour-blind than males?

Answer 35

As the condition is X-linked/ gene is found on the X chromosome, so females need 2 recessive alleles but males only need 1.

Question 36

Epistasis

Answer 36

Where different genes control the same characteristic, and the expression of 1 gene affects the expression of another.

Question 37

Examples of epistasis

Answer 37

Metabolic pathways - stages controlled by enzymes | Fur colour and fur length

Question 38

What is the chi squared test used to test?

Answer 38

whether deviation between theoretical and experimental ratios/ outcomes for genetic crosses are significant or not.

Question 39

Why are the ratios different to dihybrid inheritance when genes are linked? e.g. cross between 2 GgNn individuals,

Answer 39

Genes on same chromosome so GgNn individual produces mainly GN and gn gametes. Crossing over produces a few Gn and gN gametes, so there are fewer Ggnn and ggNn individuals.

Question 40

What type of data is the chi squared test used for?

Answer 40

categorical

Question 41

What type of epistatic allele would give these ratios in a dihybrid cross? 9: 3:4 12: 3:1

Answer 41

recessive | dominant