4.3: Inheritance

Gene

A physical unit of heredity that occupies a specific locus and codes for a specific polypeptide.

Allele

Different versions of the same gene.

Homozygous

Both alleles are the same for that gene.

Heterozygous

The alleles for that gene are different.

Genotype

All the alleles an individual contains for a specific characteristic.

Phenotype

The expression of the genotype in a specific environment - the observable characteristic.

Dominant

A gene that is always expressed when present.

Recessive

A gene that can only be expressed when homozygous.

Monohybrid

Inheritance of a single gene.

Co-dominance

Occurs when there are no dominant alleles. When both are expressed individually, such as a ginger and black cat having a calico kitten.

Incomplete dominance

Occurs when there are no dominant alleles. The alleles reach an intermediate, such as a white and red carnation having a pink carnation.

Dihybrid

Inheritance of two unliked genes.

Testcross

Test to prove the genotype of an individual expressing a dominant gene.

Linked

Genes that are on the same chromosome and cannot be separated during meiosis.

Parental

Genotypes of the original chromosome without crossing over. Most common.

Recombinant

Genotypes after crossing over. Quite rare, does not occur in most cells.

Crossover value

Percentage of linked genes that experience crossing over.

Hermaphrodite

Produce both male and female gametes, examples are angiosperms and the phylums Mollusca and Annelida.

Monoecious

Plants that have separate male and female flowers on the same plant.

Dioecious

Plants that have separate male and female individuals.

Sequential

Type of hermaphrodites that change their sex throughout their lives, such as clownfish and common slipper limpets.

Karyotype

Arrangement of homologous pairs in decreasing size order.

Autosomes

Homologous pairs with identical genes, 22 out of 23.

Pseudoautosomal

Regions where XY chromosomes are homologous - PAR1 and PAR2.

SRY

Gene chromosome Y has that X doesn’t - the region that determines the sex as male. Switches genes on the other chromosomes which are responsible for expressing male characteristics.

Homogametic

Identical sex chromosomes in gametes, happens in females.

Heterogametic

Different sex chromosomes in gametes, happens in males. Half of sperm is female, half is male.

Homologous

A pair of chromosomes, normally one is from the father, and one is from the mother.

Locus

The position of a gene on a chromosome.

Centremere

The middle part of a chromosomes, joining together the chromatids.

Chromatid

One of the two identical halves of a chromosome, replicated ready for cell division. Joined by a centromere.

Heterosomes

The sex chromosomes - chromosomes of different sizes. The 23rd pair.

Mutations

Change in the amount, structure, or arrangement in the hereditary material of an organism; DNA, or RNA with some viruses. They are spontaneous and random, and only inheritable if in the gametes. Often recessive, and rarely beneficial.

Ionising radiation

In X-rays and UV light and increases mutation rate. Radiation joins adjacent pyrimidine (T and C) bases in a DNA strand, meaning incorrect amino acids are added. Especially mutagenic at wavelength 260 nm.

Gene

Mutations that occur when DNA is not accurately copied during S phase, which is pre-cell division. There are 5 types, and can vary in effects from silent to disease inducing.

Addition

Gene point mutation where a base is added. If this happens in 3 places, an extra amino acid will be added at translation.

Subtraction

Gene point mutation where a base is deleted. If this happens in 3 places, one less amino acid will be added at translation.

Duplication

Gene point mutation where the same base is added twice.

Substitution

Gene point mutation where a different base is incoporated.

Inversion

Gene point mutation where adjacent bases on the same DNA strand swap places.

Silent

Gene point mutation where the new codon codes for the same amino acid, meaning there is no change to the polypeptide.

Sickle cell anemia

A condition where the DNA triplet CTC is substituted for CAC, adding valine over glutamate. This causes red blood cells to be sickle shaped. This is a dominant gene, and can be expressed with co-dominance.

Aneuploidy

When a whole chromosome is lost or added. Occurs during non-disjunction, where a faulty spindle causes chromosomes to be shared unequally.

Non-disjunction

Occurs during meiosis, either anaphase 1 or 2, when chromosomes or chromatids separate unequally due to a faulty spindle. One daughter cell has two copies of a chromosome, and one has none. Causes aneuploidy.

Chromosome

Mutations that occur when chromosomes do not accurately join to their homologous partners. Gametes can fuse to produce an organism, but further meiosis is impossible.

Translocation

When a fragment of one chromosome attaches to another. An example is ——— Down’s syndrome, where a fragment of C21 attaches to C14.

Euploid

Cells with complete sets of chromosomes.

Endomitosis

The replication of chromosomes not followed by cytokinesis. If this happens in an early embryo, tetraploid cells are produced. This can occur with infertile triploids, which would make them fertile. More common, and beneficial, in flowering plants, due to them being asexual hermaphrodites.

Carcinogen

An agent that causes cancer, makes up some mutagens.

Tumour suppressor

Term for genes that regulate mitosis and prevent cells from dividing too quickly. Mutations to these genes allow the cell to undergo repeated and rapid mitosis, characterising cancer.

Metastases

Secondary tumours, formed when tumours spread around the body and invade other tissues. These are malignant.

TP53

Tumour suppressor gene which produces p53 protein. Abnormalities in this gene are found in more than half of all human cancers. It triggers repair of damaged DNA, prevents cell from entering S phase while DNA is repaired, and if unrepairable initiates apoptosis.

Proto-oncogene

A gene that codes for a protein which contributes to cell division. If the gene has an extra copy or a mutation moves the gene next to a gene which permanently switches on the gene, rapid and repeated mitosis occurs, causing cancer.

Polycyclic hydrocarbons

A chemical in tar, which collects in lungs. It enters alveolar cells and slides between DNA base pairs, causing mutations by preventing accurate replication.

Epigenetics

The study of how the behaviour and the environment can alter gene functioning. Examples include DNA methylation and histone acetylation.

Methyl

If this compound is heavily added to cytosine, it makes it less likely to be transcribed. Normally, hydroxymethyl is added instead. If added to lysine, it spreads out nucleosomes, increasing transcription and translation.

Acetyl

When this attaches to the amino acid lysine, it changes histone interaction with DNA, causing nucleosomes to be more spread out. This allows them better access to enzymes, and transcription is increased.

Genomic imprinting

When genes are inactivated in gametes, transferring this to any offspring, often occurring due to DNA methylation. Examples are proto-oncogenes and tumour suppressor genes. This can cause medical conditions.

Burr body

Term for the inactivated X chromosome in female mammals, which becomes a mass of densely staining chromatin. Tortoiseshell cat patterns show X inactivation.

Lysine

An amino acid which if added to by an acetyl or methyl group, nucleosomes become modified, meaning the coiling becomes looser and enzymes have better access. An epigenetic modification.

Glutamate and valine

The amino acid which is coded for by CTC, and the amino acid coded for by CAC respectively. The latter is small and hydrophilic, causing the cell membranes of red blood cells to collapse and become sickle shaped, causing sickle cell anemia.

Acridine

A mutagen which slides between base pairs in the double helix, preventing DNA polymerase from inserting the correct nucleotide at replication.