Introduction to Genetics Chapter 11-1 & 11-2: The Work of Gregor Mendel <>

A GLIMPSE AT THE HUMAN GENETIC CODE ■ The human body has 100 trillion cells. Each cell (except blood cells) contains the human genome -- all the genetic information necessary to build a human being. ■ Six feet of DNA are packaged into 23 pairs of chromosomes (one from each parent in each pair) in the cell nucleus. ■ Each of the 46 human chromosomes contains the DNA for thousands of individual genes or units of heredity. ■ DNA in each gene contains four chemical bases that hold recipes for making all organisms. They are: A (adenine), T (thymine), G (guanine) and C (cytosine). A pairs with T and G pairs with C. ■ Proteins, made of amino acids, are the essential ingredients of all organs and chemical activities. Their function depends on their shapes, which are determined by the 50,000 to 100,000 genes in the cell nucleus. ■ Source: National Human Genome Research Institute

Genetics- the study of heredity

Mendel’s Pea Plants ■ Mendel was born in 1822 in what is now the Czech Republic ■ He studied science & mathematics before he became a priest & teacher ■ Mendel worked as a priest and gardener for the monastery ■ He worked with pea plants

Why Pea Plants? ■ easy to cultivate ■ small in size ■ reproduce quickly ■ large amount of offspring ■ Have 7 observable “either or” traits ■ Pea plants are also able to reproduce in two ways: self-fertilize and cross- fertilize

Humans have a few “either or” traits but, most of our traits come in a wide range or variety.

Pea plants have 7 “either or” traits Possible Phenotypes

Pea plants have flowers. Flowers contain all the reproductive organs. Pollen is plant sperm.

Reproduction in Flowers ■ Flowers have both male and female reproductive parts ■ Self-pollinating ■ Cross-pollinating

Self-Pollination ■ Self-pollinating plants can fertilize themselves ■ Offspring are identical to the parent if the plant is a pure or true breed

Mendel let his plants self-pollinate to find the true-breeding plants

Cross-pollination ■ Cross-pollinators are not self-fertilizing ■ they are fertilized by the pollen of another flower ■ produce offspring that are a mix of the parents called hybrids

Making Pea Plants Unable to Self-pollinate & then Cross-Pollinating them Mendel could choose which plants reproduce cannot produce pollen anymore.

Genes & Dominance: 7 “either or” traits Possible Phenotypes

Generations ■ P= parental ■ parents ■ F1= 1 st filial ■ children ■ F2= 2 nd filial ■ Grand children ■ F3= 3 rd filial ■ Great grand children

STOP. Part 2 on remaining slides :-)

Mendel’s Experiment ■ Monohybrid cross ■ P generation = Crossed two true-breeding plants with different traits ■ Purple x White ■ F1 generation= was not a mix, but resembled one parent ■ All Purple Phenotype

Conclusions ■ That traits were controlled by genes ■ Gene for flower color ■ Genes come in many varieties called alleles ■ Ex. Purple flower allele (P) or white flower allele (p) ■ Some alleles are dominate and others are recessive ■ Principle of dominance ■ If an organism inherits the dominant allele than it will show the dominant trait ■ PP, Pp, pP =purple but pp=white ■ What happens to the recessive? Is it hiding or did it disappear? P p

Another Experiment ■ Mendel allowed the F1 generation to self-fertilize ■ F1 generation= ■ All Purple (dominant phenotype) ■ F2 generation = ■ Most plants expressed the dominant trait purple phenotype ■ Some expressed the recessive trait white phenotype ■ How did the recessive separate from the dominant?

Approximately 3:1 in F2

The Law of Segregation ■ Body cells contain 2 alleles for a gene ■ Gametes, sperm & eggs, contain 1 allele for a gene, so each parent can only pass on ½ of their chromosomes ■ Offspring’s cells contain 2 alleles for a gene 1 from each parent PpPp P p P PpPp P p PpPppPpPp

Diploid (2n) vs. Haploid (n) In humans n = 23 ■ Human body cells have 23 pairs of chromosomes ■ Diploid = 2 x 23 ■ Gametes, sperm and egg, have 23 individual chromosomes ■ Haploid = 23 ■ Offspring are produced when sperm and egg donate chromosomes ■ Diploid = 2 x 23

Phenotype vs. Genotype ■ Phenotype- physical characteristics ■ Genotype- the genetic makeup of the alleles ■ Homozygous dominant PP ■ Homozygous recessive pp ■ Heterozygous Pp ■ Examples: ■ Purple Phenotypes ■ PP – homozygous dominant genotype ■ Pp – heterozygous genotype ■ White Phenotype ■ pp – homozygous recessive genotype

Punnett Square Predicts Probability ■ Lets say that: ■ Mother ■ Phenotype: Purple ■ Genotype: PP homozygous ■ Father ■ Phenotype: Purple ■ Genotype: PP homozygous ■ Offspring ■ Phenotype: 100% Purple ■ Genotype: 100% PP homozygous PP P P P P PP P P PP

Punnett Square Predicts Probability ■ Lets say that: ■ Mother ■ Phenotype: Purple ■ Genotype: Pp heterozygous ■ Father ■ Phenotype: Purple ■ Genotype: Pp heterozygous ■ Offspring ■ Phenotype: 75% purple 25% white ■ Genotype: 1:2:1 P Pp p pp p p PP P P

Punnett Square Predicts Probability ■ Lets say that: ■ Mother ■ Phenotype: Purple ■ Genotype: Pp heterozygous ■ Father ■ Phenotype: White ■ Genotype: pp homozygous ■ Offspring ■ Phenotype: 50% purple 50%white ■ Genotype: 1:1 p p p p p p p p p P P P

Punnett Square Predicts Probability ■ Lets say that: ■ Mother ■ Phenotype: White ■ Genotype: pp homozygous ■ Father ■ Phenotype: White ■ Genotype: pp homozygous ■ Offspring ■ Phenotype: 100% white ■ Genotype: 100% pp homozygous recessive p p p p pp p p p p p p