Patterns of inheritance: Mendel and beyond

Contrasting characters in peas

Experiment with garden peas I

In this case a true-breeding tall plant was crossed with a true-breeding short plant. All of the plants in the next generation were tall.

These are the results Mendel obtained when he crossed two heterozygotes.

‘Paint-pot’ theory of inheritance

Mendel’s finding was contrary to the prediction of the ‘Paint-pot’ theory of inheritance

Mendel’s 1st law- law of segregation

Mendel’s law of segregation -explanation of monohybrid ratio

Mendel described patterns of inheritance in the 1860s, but it wasn’t until the early 1900s that inherited traits, genes, were linked to cellular structures called chromosomes. The number of chromosomes varies among species, but all chromosomes contain genes arranged linearly at specific locations, called loci.

This is how geneticists today represent the cross from the previous screen. The original plants are the P generation and their offspring are the F1 generation. The T and t symbols represent dominant and recessive alleles of a single gene.

we breed the offspring to one another in the cross Tt x Tt, and wish to predict the results we need to follow the production of gametes during meiosis. These individuals make gametes with either T or t in equal numbers.

The chance of gametes of different types encountering one another is represented on a Punnett square. The genotypic ratio of homozygous dominant: heterozygous: homozygous recessive individuals is 1:2:1.

Many genetics problems will fall in the category of a monohybrid cross Many genetics problems will fall in the category of a monohybrid cross. They all can be approached using the model above

Testcross. a heterozygote was crossed with a homozygous recessive individual. A situation in which an individual of dominant phenotype, but of unknown genotype, is crossed with one or more recessive individuals. This can provide information on the unknown genotype.

Dihybrid cross

Dihybrid cross

Dihybrid cross The inheritance of two contrasting characters were considered at the same time

Law of independent assortment

Mendel’s 2nd law

Mendel’s 2nd law explained by observing movement of chromosomes at meiosis

Cell division

The cell cycle

Interphase

Interphase

Mitosis – division of the nucleus

Prophase metaphase anaphase telophase Division of the nucleus Prophase metaphase anaphase telophase

Division of the cytoplasm

Produce identical daughter cells

Mitosis and cell cycle

Meiosis Reduction division Generate variability in gametes

Meiosis-Interphase

Meiosis-prophase I

Meiosis-metaphase I

Meiosis-anaphase I

Meiosis-telophase I

Meiosis I -animated

Meiosis II prophase II metaphase II anaphase II telophase II

Meiosis II -animated

Meiosis-overall

Independent assortment

Crossing over

Anaphase I and II note new gene combinations as a result of crossing over

Comparing Mitosis and Meiosis

Comparing Mitosis and Meiosis

Incomplete dominance In some cases, alleles may not interact in a dominant/recessive pattern. Heterozygotes have an intermediate phenotype or express both alleles. The genotypic ratio will match the phenotypic ratio. Snapdragons demonstrate incomplete dominance.

Incomplete dominance II

Multiple allele

Linkage and crossing-over

Recombinant frequency

Skin colour_a pair of twin sisters

Continuous or discontinuous

Polygenic effects on genetic variation

Polygenic inheritance_skin colour

Skin colour is a continuous variation because…

How would U explain the very different skin colour of the twin sisters?

X-inactivation

X-inactivation