Genetics Key Concepts: genes, inheritence, disorders

Gene

DNA sequence that encodes for a protein (gene product)

Allele

A possible variation of a single gene

Trait

Manifestation of the expression of one or more genes together in a cell or tissue

Monogenic traits

Single gene controlled traits, rare in humans (e.g. freckles)

Polygenic traits

Traits controlled by multiple genes, most human traits (e.g. eye colour, skin colour)

Heredity

The passing of traits from parents to offspring

Independent assortment

Each trait is separate and inherited independent of other traits

Mendelian genetics

Study of heredity based on the principles proposed by Gregory Mendel

Genotype

The genetic make up; which alleles the individual has for a gene of interest

Phenotype

The expression of the genotype; physical characteristic

Homozygous

Two alleles the same (e.g. A/A or a/a)

Heterozygous

Two different alleles (e.g. A/a)

Dominant allele

An allele which masks the phenotype of another allele, usually denoted as an upper-case letter

Recessive allele

The allele that gets masked, usually denoted as a lower-case letter, not observed in the phenotype

Punnett square

A tool used to predict the offspring based on parents' genotypes

Freckles

A dominant trait that will result in visible freckles with only one copy of the freckle gene

Homozygous dominant genotype

Genotype represented as F/F

Heterozygous genotype

Genotype represented as F/f, carriers of the recessive gene

Homozygous recessive genotype

Genotype represented as f/f

Freckled phenotype

Can only be expressed if the genotype is F/F or F/f

No freckles

Recessive trait represented by f.

Probability of offspring

Likelihood of children having specific traits.

Independent event

Each child is an independent event in genetics.

X-linked inheritance

Traits determined by genes present on sex chromosomes.

X chromosome

Has approximately 1,500 genes.

Y chromosome

Has approximately 200 genes.

X-linked disorders

Usually recessive traits linked to the X chromosome.

Gene for red-green colour blindness

An example of an X-linked disorder.

Female phenotype

Genotype represented as X/X.

Male phenotype

Genotype represented as X/Y.

Carrier mother

A mother with one normal vision allele and one colour blindness allele.

Normal colour vision father

A father with two normal vision alleles.

Homozygous dominant offspring

All offspring possibilities have heterozygous genotype.

Heterozygous dominant offspring

Half possible offspring are heterozygous dominant, half are homozygous recessive.

Key words in genetics problems

Words like dominant, homozygous, etc., that help determine traits.

XCB

normal colour vision

Mother genotype

XCB/Xcb

Father genotype

XCB/Y

Dominant-recessive inheritance

A genetic inheritance pattern where one allele masks the expression of another.

Incomplete dominance

Both alleles contribute to the phenotype, resulting in an intermediate phenotype.

Multiple alleles

More than two possible alleles for a gene, leading to various combinations in the population.

Codominance

Both alleles express a gene product simultaneously.

ABO blood grouping

Blood grouping assigned based on RBC surface antigens with three possible alleles: IA, IB, and i.

IA

gene for A antigen

IB

gene for B antigen

i

does not produce a gene product and is a recessive allele

Huntington's disease

A late onset neurodegenerative disorder characterized by progressive disease with involuntary movements and dementia.

Mean age of onset for Huntington's disease

35 - 44 years old

Monogenic disorders

Disorders caused by alteration of a single gene.

Chromosome disorders

Disorders involving entire chromosomes or large segments that are missing, duplicated, or inserted.

Multifactorial disorders

Disorders caused by a combination of genetic and environmental factors.

Genetic disorder

A disorder that occurs partly or wholly due to abnormalities within an individual's genome.

Environmental influence

Factors from the environment that can affect genetic expression and development of disorders.

Autosomal dominant inheritance

The phenotype is observed in most generations with each affected person usually having an affected parent.

Autosomal recessive inheritance

The phenotype can skip generations, with affected individuals possibly having unaffected parents.

X-linked recessive inheritance

Never transmits from father to son, mainly affects males, and daughters are usually carriers.

Haemophilia

An example of an X-linked recessive disorder.

Genetic testing

A process that can identify genetic disorders and often requires genetic counselling.

Genetic counselling

A service that provides information and support to individuals undergoing genetic testing.