Intro to genetics and human genetic diseases

Mutation

Permanent alteration of a gene

Allele

An alternative form of a gene

Homozygous

Two identical alleles for the same gene

Heterozygous

Two different alleles for the same gene

Genotype

The alleles carried by the individual

Phenotype

The observable traits of an individual

True-breeding

Organism that possess a phenotype that is always transmitted to the off spring

Haploid

A genome that contains a single set of chromosomes

Example of Haploid

Prokaryotes, meiotic simple eukaryotes like yeast, germ cells, gametes

Locus

A location of a particular gene

Diploid

A genome that contains paired chromosomes

Example of diploid

Most somatic cells of most animals and some plants like garden peas, human zygotes.

Law of Dominance

In a cross of parents that are true-breeding for contrasting phenotypes, only one phenotype is expressed in the offspring- the dominant phenotype.

F0 generation

Parents

F1 generation

Hybrids

If parents are true-bred homozygous all F1 hybrids are heterozygous or homozygous?

Heterozygous as well as having the same genotype. All their phenotype are dominant.

Do all phenotypes follow the dominant/recessive pattern?

No

Law of segregation

Parental alleles are segregated into different haploid gametes with equal probability.

In F1 self-fertilization occurs, what is the ratio of dominant to recessive allele being expressed?

The recessive phenotype is expressed in F2 generation, 3:1

Frequency of genotypes in F2 generation? (F1 are all heterozygous)

• ¼ Homozygous dominant

• 2/4 Heterozygous

• ¼ Homozygous recessive

Frequency of phenotypes in F2?

• ¼ recessive

• ¾ dominant

  Mendelian ratio of a hybrid cross is 3: 1

Law of independent assortment (4 letters)

Genes for different phenotypes are sorted separately from one another so that the inheritance of one phenotype is not dependent on the inheritance of another.

In independent assortment the gametes have how many alleles? To make zygotes with how many alleles?

2 alleles in a gamete to make 4 alleles in a zygote.

What is the phenotypic ratio of independent assortments?

9:3:3:1

What causes dependent assortment?

When recombination does not occur between the two genes located very close to each other during meiosis.

What causes Human Mendelian (monogenic) diseases.

A mutation in a single gene and can be present on one or both chromosomes.

Autosomal dominant

If one parent have the disease, the offspring will have the disease. If both parents do not then disease is NOT dominant. (Huntington’s disease)

Autosomal recessive

Both parents needs to be heterozygous carriers for offspring to have disease.

Disease can skip generations. (Cystic fibrosis)

X-linked recessive

Mother must be at least a carrier for offspring to have disease.

Sex bias in the preference of disease: more frequent in males. (Duchenne muscular dystrophy)

Polygenic diseases

Several alleles acting together contribute to occurrence and severity of the disease

Genetic heterogeneity

Many mutations may result into the same disease (ie. retinitis pigmentosa caused by mutations in any one of 60 diff genes)

Mechanism of Law of segregation

• Parental alleles are randomly separated, sex cells contain only one allele of the pair

• Offspring inherit 1 genetic allele from each parent in a zygote

• parental alleles are segregated into different haploid gametes with equal probability

Mechanism of law of independent assortment: Random Chromosome Assortment

Mechanism of law of independent assortment: recombination

The alleles of genes A, B and C will be inherited independently of the alleles in gene D and E. Notice that the chromosome of each gamete has a different combination of alleles.