Mendelian Genetics: Principles of Inheritance

Mendelian Principles of Inheritance

Gregor Mendel

• In the 19th century, he studied inheritance: the passing of characteristics from parents to offspring.
• His work was based on the study of the crossing of different varieties of plants.
• The species chosen was the pea (Pisum sativum).

Method

• Characteristic selection
• Obtaining pure-bred individuals for those characteristics

Parental generation (P)

• Crossing two different pure breeds to obtain descendants

First filial generation (F1)

• Crossing F1 individuals and studying their descendants

Second filial generation (F2)

Complete Inheritance

In the heterozygote, the dominant phenotype is manifested.

Special Cases in Genetics

Complete Inheritance

In the heterozygote, the dominant phenotype is manifested.

Intermediate Inheritance

The phenotype of the heterozygote is a mixture of phenotypes of pure-breds.

Co-dominance

The phenotype of heterozygotes shows the phenotypes of both pure-breds.

Multiple Alleles

Sometimes there are more than two alleles for one characteristic.

Gene Interaction

Sometimes a pair of alleles influences the expression of another different pair of alleles, resulting in unexpected phenotypes in offspring.

Lethal Genes

They produce the death of the individual and modify the usual proportions of phenotypes and genotypes in offspring.

Quantitative Inheritance

Sometimes there are many alternatives for a characteristic: there are many phenotypes.

The Inheritance of Sex

The Inheritance of Sex

Sex Chromosomes

• XX/XY system
• Pair 23 of human chromosomes
• Each gamete carries a sex chromosome
  • Ovules: X chromosome
  • Sperm: X chromosome or Y chromosome
• Sex depends on the sex chromosome of the spermatozoid.

Inheritance Linked to Sex

• Sex chromosomes carry sex genes and other genes that are unrelated to sex.

Y Chromosome

• Small, with few genes
• Any characteristic located on this chromosome will be inherited by male children.

X Chromosome

• Large, with many genes that are not on the Y chromosome: located in the differential segment.
• The characteristics in these genes are called sex-linked or X-linked characteristics.

Inheritance Linked to Sex

• Inheritance of characteristics linked to the differential segment of the X chromosome.
• In men, all genes located in the differential segment will manifest, whether dominant or recessive: they are neither homozygous nor heterozygous.
• Females carry two alleles for this trait: they are homozygous or heterozygous (carriers).

X-linked characteristics are much more common in men.

Examples of X-linked diseases:

• Colour blindness
• Hemophilia
• Duchenne muscular dystrophy

Inheritance Influenced by Sex

• Genes for which an allele is dominant in one sex and recessive in the other.
• Located on the homologous segment of the sex chromosomes.
• Example: Alopecia