Key words

Question 1

PREFORMATION

Answer 1

The belief that an embryo is a miniature adult (homunculus). There is no distinction between different stages of life

Question 2

EPIGENESIS

Answer 2

The belief that adult forms arise by development from different forms. The embryo contains information specifying a developmental programme

Question 3

MENDEL’S 1ST LAW

Answer 3

Segregation - there are two copies of each ‘particle’, one from each parent. They remain unaltered and one is passed on at random in each sperm/egg

Question 4

MENDEL’S 2ND LAW

Answer 4

Independent assortment - traits are inherited independently

Question 5

MENDEL’S 3RD LAW

Answer 5

Dominance - the inherited ‘particles’ may be either a) dominant or b) recessive

Question 6

ALLELE

Answer 6

A variant form of a given gene, meaning it is one of two or more versions of a known mutation at the same place on a chromosome

Question 7

HOMOZYGOUS

Answer 7

An individual carrying two identical alleles

Question 8

HETEROZYGOUS

Answer 8

An individual carrying two different alleles

Question 9

GENOTYPE

Answer 9

The specific allele composition of an individual

Question 10

PHENOTYPE

Answer 10

The observable properties of an individual

Question 11

MITOSIS

Answer 11

A type of cell division that results in two diploid daughter cells each having the same number of chromosomes as the parent nucleus, typical of ordinary tissue growth

Question 12

MEIOSIS

Answer 12

A type of cell division that results in four haploid daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores

Question 13

PLEIOTROPY

Answer 13

When one gene influences two or more seemingly unrelated phenotypic traits

Question 14

POLYGENIC INHERITANCE

Answer 14

Occurs when one characteristic is controlled by two or more genes. Often the genes are large in quantity but small in effect. Examples of human polygenic inheritance are height, skin color, eye color and weight

Question 15

COMPLEMENTATION TEST

Answer 15

A test to establish the relationship between 2 recessive mutations by checking the phenotype of the offspring. If the parental mutations are at the same loci, the progeny are affected

Question 16

ADDITIVE EFFECTS OF POLYGENIC INHERITANCE

Answer 16

Each allele of each gene has an additive effect e.g. each contributing allele produces one unit of colour

Question 17

EPISTASIS

Answer 17

The effect of a gene mutation is dependent on the presence or absence of mutations in one or more other genes

Question 18

RECESSIVE EPISTASIS

Answer 18

Two alleles have to be inherited in order for the phenotype of the second gene to be masked e.g. dog coat colours

Question 19

DOMINANT EPISTASIS

Answer 19

One allele of a gene has to be inherited to mask the alleles of another gene e.g. squash colouration

Question 20

CODOMINANCE

Answer 20

Neither allele can mask the expression of the other allele e.g. blood groups

Question 21

INCOMPLETE DOMINANCE

Answer 21

When a dominant allele, or form of a gene, does not completely mask the effects of a recessive allele, and the organism’s resulting physical appearance shows a blending of both alleles e.g. snapdragons

Question 22

UNDERDOMINANCE

Answer 22

The selection against the heterozygote, causing disruptive selection and divergent genotypes. It exists in situations where the heterozygotic genotype is inferior in fitness to either the dominant or recessive homozygotic genotype

Question 23

OVERDOMINANCE

Answer 23

Where the phenotype of the heterozygote lies outside the phenotypical range of both homozygous parents / heterozygous individuals have a higher fitness than homozygous individuals e.g. Sickle cell anaemia

Question 24

VARIABLE EXPRESSIVITY

Answer 24

The degree in which a genotype is phenotypically expressed. For example, multiple people with the same disease can have the same genotype but one may express more severe symptoms, while another carrier may appear normal e.g. Tuberolis Schlerosis

Question 25

INCOMPLETE PENETRANCE

Answer 25

The likelihood that a clinical condition will occur when a particular genotype is present. A condition is said to show incomplete penetrance when some individuals who carry the pathogenic variant express the associated trait while others do not e.g. Alzheimer’s disease

Question 26

LINKAGE

Answer 26

The tendency of genes that are close together on a chromosome to be inherited together during meiosis

Question 27

RECOMBINATION FRACTION (Rf)

Answer 27

100% x no. of recombinants / no. of offspring

Question 28

TWO POINT TEST CROSS

Answer 28

Determines the recombinant frequency between 2 linked genes

Question 29

DIHYBRID CROSS

Answer 29

A cross between two different genes that differ in phenotypes

Question 30

LINKAGE MAP

Answer 30

A map of the genes on a chromosome based on linkage analysis. A linkage map does not show the physical distances between genes but rather their relative positions, as determined by how often two gene loci are inherited together.

Question 31

THREE POINT TEST CROSS

Answer 31

Used to determine the loci of three genes in an organism’s genome. An individual heterozygous for three mutations is crossed with a homozygous recessive individual, and the phenotypes of the progeny are scored