Mendelian Genetics

Homologous chromosomes

A pair of chromosomes (one from each parent) that determine the same characteristics eg. identical in length, centromere position and gene loci, and pair with each other during prophase I of meiosis but need not be genetically identical.

Gene

A specific sequence of nucleotides along a DNA molecule which codes for a specific amino acid in a polypeptide; the fundamental unit of inheritance.

Gene locus (plural: loci)

A fixed position of a gene on a particular chromosome

Allele

An alternative form of a gene occupying the same gene locus on homologous chromosomes; may be dominant, recessive or codominant; only one of each pair of alleles is represented in a gamete.

Gene pair

The two copies (alleles) of a particular gene present in a diploid cell.

Homozygous (individual: homozygote)

Having identical alleles of a gene pair (e.g. AA or aa)

Heterozygous (individual: heterozygote)

Having two different alleles of a gene pair (e.g. Aa)

Dominant allele

An allele that is always expressed in the homozygous or heterozygous condition and shows complete dominance over the other alleles for the same gene

Recessive allele

An allele expressed in the phenotype only in the homozygous condition and is masked in heterozygotes

Phenotype

The observable characteristics of an individual, dependent on its genotype and sometimes the environment.

Genotype

The combination of alleles of an organism that produces its phenotype.

Cross

The deliberate mating of two organisms in genetic analysis.

Selfing

Fusion of male and female gametes from the same organism (used for plants only); used to produce a pure line.

Pure line (true breeding)

Members of a pure line are homozygous at the gene loci in question and are said to breed true, either homozygous dominant or homozygous recessive, never heterozygous

Sibling mating

Mating between siblings.

F1 generation (First filial generation)

The first generation of offspring that results from a pair of controlled initial parents

F2 generation (Second filial generation)

Only applicable for a generation obtained by selfing or sibling-mating the F1 generation.

Offspring

General term applicable for a generation regardless of whether parents are heterozygous or homozygous for he trait

Monohybrid inheritance

Involves the inheritance of one characteristic controlled by alleles of one gene.

phenotypic ratio for pure breeding x pure breeding

F1 generation: all homozygous dominant

F2 generation: 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive

Phenotypic ratio for Tt x tt

1:1

Test Cross

It is a cross between an organism with unknown genotype with one which is homozygous recessive at the gene locus in question to deduce the unknown genotype from the phenotypes of the offspring

Law of Segregation (Mendel’s first law)

Two alleles of a gene pair segregate into the gametes so that half of the gametes carry only one allele of the gene pair while the other half carries the other allele of the gene pair.

Incomplete dominance

A condition where the heterozygote exhibits a phenotype that is an intermediate between the two homozygous forms. It occurs when a single dominant allele does not make enough gene product to control the characteristics completely

Biochemical basis for incomplete dominance

The dominant allele(CR), when present in only one copy per cell, produces insufficient enzyme to synthesize enough red pigment (phenotype). The allele CW codes for a non-functional enzyme.

Codominance

A condition where both alleles are expressed equally in the phenotype of the heterozygote (eg. MN blood group system in humans)

Biochemical basis for co-dominance

In co-dominance, both alleles at a locus produce functional gene products that will appear in the heterozygote.

Multiple alleles

A gene that controls a single characteristic with more than two alleles eg. ABO group system

Antigen (blood typing)

A surface protein (A or B) on red blood cells that can trigger antibody production to bind to foreign molecules during blood transfusion

MN blood group system

Human blood group determined by codominant alleles LM and LN producing M, N, or both proteins on RBC membranes.

Sex chromosomes

The X and Y chromosomes responsible for sex determination by the 23rd pair in humans (XX = female, XY = male).

Sex-Linked Genes

Genes that are found on the sex chromosomes

X vs Y chromosomes

Y chromosome is a smaller chromosome than the X chromosome hence there are genes present on the X chromosomes that are absent on the Y chromosome

X-Linked recessive disorders

if the male inherits a X chromosome with the recessive allele, he will suffer from the disorder as there is no corresponding Y chromosome to mask the recessive allele

if a female inherits a X chromosome with the recessive allele and a X chromosome with the dominant allele from the other parent, she will not suffer from the disease (carrier)

eg. haemophilia and red-green colour blindness

Autosome

Any chromosome that is not a sex chromosome.

Haemophilia

An X-linked recessive disorder in which blood fails to clot properly.

Red-green colour blindness

An X-linked recessive condition causing inability to distinguish red from green shades.

Observed ratios in the offspring often differ from the expected ratio especially when there are small numbers of offspring as…

1. expected ratios only indicate the probability of producing the genotypes/phenotypes in offspring

2. fertilisation of the sperms with ova is a random event

Pedigree chart

A diagram showing the inheritance of traits across generations in a family, used to deduce genetic patterns of a characteristic.

Possible for organisms like fruit flies or pea plants as they have short life cycles and produce many offspring, unlike humans with long life cycles, do not produce many offspring per generation and is ethically unacceptable to carry out experimental crosses with humans

Autosomal dominant inheritance

Pattern where a dominant allele on an autosome causes a trait to appear in every generation and in affected parents.

Autosomal recessive inheritance

Pattern where a recessive allele on an autosome causes a rare trait that can appear in offspring of unaffected parents.

Gamete

A haploid reproductive cell (sperm or egg) that carries one allele of each gene.