A-Level Biology: Genetics and Inheritance Flashcards

Question 1

Define Homologous Chromosomes.

Accepted Answer

A pair of chromosomes, one inherited from each parent, that carry the same genes at the same loci but may carry different alleles of those genes.

Question 2

What is Autosome?

Accepted Answer

Any chromosome that is not a sex chromosome. Humans have 22 pairs of autosomes and one pair of sex chromosomes in each body cell.

Question 3

What is Hemizygous?

Accepted Answer

Having only one copy of a gene instead of a pair, as human males do for genes carried on their single X chromosome.

Question 4

What is Pure-Breeding?

Accepted Answer

Describes organisms that are homozygous for a characteristic, so breeding two of them together always produces offspring with that same phenotype.

Question 5

Define F1 Generation.

Accepted Answer

The first generation of offspring produced by crossing two parent organisms, usually two pure-breeding parents with contrasting characteristics.

Question 6

Define F2 Generation.

Accepted Answer

The second generation of offspring, produced by crossing two members of the F1 generation with each other or by self-pollinating an F1 individual.

Question 7

Define Monohybrid Cross.

Accepted Answer

A genetic cross that follows the inheritance of one gene with two alleles, used to predict the genotypes and phenotypes of the offspring.

Question 8

Define Dihybrid Cross.

Accepted Answer

A genetic cross that follows the inheritance of two different genes at the same time, each gene having its own pair of alleles.

Question 9

Define Multiple Alleles.

Accepted Answer

A gene that has more than two different alleles in a population, although each diploid individual can carry only two of them.

Question 10

Define Lethal Allele.

Accepted Answer

An allele that causes the death of an organism, often before birth, when present in a particular genotype, which changes the offspring ratios expected from a cross.

Question 11

Define Pedigree Diagram.

Accepted Answer

A chart that shows a family's relationships across several generations and which members have a particular characteristic, used to work out how it is inherited.

Question 12

Define Crossing Over.

Accepted Answer

The exchange of sections of DNA between non-sister chromatids of a homologous pair during prophase I of meiosis, creating new combinations of alleles.

Question 13

What is Chiasma?

Accepted Answer

The point where non-sister chromatids of a homologous pair remain joined during prophase I of meiosis, marking where crossing over has occurred. The plural is chiasmata.

Question 14

Define Independent Assortment.

Accepted Answer

The random orientation of each homologous pair on the equator during metaphase I of meiosis, so each gamete receives a random mix of maternal and paternal chromosomes.

Question 15

Define Autosomal Linkage.

Accepted Answer

The situation where two or more genes sit close together on the same autosome, so their alleles usually travel into gametes together instead of assorting independently.

Question 16

Define Recombinant Offspring.

Accepted Answer

Offspring that show a combination of characteristics not found together in either parent, produced when crossing over separates alleles of linked genes.

Question 17

Define Recombination Frequency.

Accepted Answer

The number of recombinant offspring divided by the total number of offspring, multiplied by 100. Genes further apart on a chromosome give higher values, up to a maximum of 50%.

Question 18

What is Epistasis?

Accepted Answer

An interaction between two genes at different loci in which the alleles of one gene mask or modify the expression of the other gene.

Question 19

Define Recessive Epistasis.

Accepted Answer

Epistasis where a homozygous recessive genotype at one gene masks the other gene, giving a 9:3:4 phenotype ratio when two double heterozygotes are crossed.

Question 20

Define Dominant Epistasis.

Accepted Answer

Epistasis where a single dominant allele at one gene masks the other gene, giving a 12:3:1 phenotype ratio when two double heterozygotes are crossed.

Question 21

Define Null Hypothesis.

Accepted Answer

In a chi-squared test on a genetic cross, the statement that there is no significant difference between observed and expected offspring numbers, so any difference is due to chance.

Question 22

Define Degrees of Freedom.

Accepted Answer

In a chi-squared test, the number of categories being compared minus one, used with the probability level to look up the critical value.

Question 23

Define Critical Value.

Accepted Answer

The chi-squared value at the chosen probability level, usually 0.05, and degrees of freedom. A calculated value larger than it means the null hypothesis is rejected.

Question 24

Define Gene Pool.

Accepted Answer

All of the alleles of all of the genes present in a population at a particular time.

Question 25

Define Allele Frequency.

Accepted Answer

The proportion of all copies of a gene in a population that are one particular allele, written as a decimal or a percentage.

Question 26

Define Hardy-Weinberg Principle.

Accepted Answer

The principle that allele frequencies in a population stay constant between generations, provided the population is large, mates randomly, and has no mutation, migration or selection.

Question 27

Define Genetic Drift.

Accepted Answer

Random change in allele frequencies from one generation to the next, caused by chance differences in which individuals survive and reproduce. Its effect is greatest in small populations.

Question 28

Define Founder Effect.

Accepted Answer

The loss of genetic diversity that occurs when a small group of individuals leaves a population to start a new one, carrying only some of the original alleles.

Question 29

Define Genetic Bottleneck.

Accepted Answer

A sharp reduction in population size, caused by an event such as hunting, disease or natural disaster, that leaves survivors with much less genetic diversity.

Question 30

Define Stabilising Selection.

Accepted Answer

Natural selection that favours individuals with average phenotypes and acts against both extremes, reducing variation around the mean.

Question 31

Define Directional Selection.

Accepted Answer

Natural selection that favours individuals at one extreme of a phenotype range, shifting the population mean in that direction over generations.

Question 32

Define Disruptive Selection.

Accepted Answer

Natural selection that favours individuals at both extremes of a phenotype range over intermediate individuals, which can split a population into two groups.

Question 33

Define Reproductive Isolation.

Accepted Answer

The situation where members of two populations can no longer interbreed to produce fertile offspring, for example because of different breeding seasons, behaviour or body structure.

Question 34

Define Allopatric Speciation.

Accepted Answer

The formation of new species after populations are separated by a geographical barrier, so different selection pressures and drift change their allele frequencies until they are reproductively isolated.

Question 35

Define Sympatric Speciation.

Accepted Answer

The formation of a new species from a population living in the same area, when reproductive isolation arises without any geographical barrier.