Mendel and beyond

Patterns of inheritance:

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. 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 byobserving

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

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

anaphase II

metaphase II

prophase 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