Mendel and Heredity

Chapter 10 and 12

Key Terms• Heredity – The passing of traits from parents

to offspring

• Genetics – the branch of science that studies heredity

• Gene – the unit of heredity – codes for a specific trait

• Gregor Mendel – the father of modern genetics

• Gametes – male and female sex cells

• Zygote – forms when the sperm and egg meet

Summary of Mendel’s Experiments

• Why peas?– Characters exist in two clear forms (tall or

short, purple or white flowers, etc.)– Self-fertilization is possible– Small plant– Grows easily– Matures quickly– Many offspring

What did Mendel do?• He self pollinated the pea plants until they were

“true-breeding”– Had only one version of a trait in their genes– Only genes for purple flowers, or for being tall– This made the “P-generation” (parent generation)

• He then crossed 2 different true-breeding plants and recording the results. This made the F1 generation (filial 1). These are hybrids.

• Finally, he crossed 2 members of the F1 generation making the F2 generation (filial 2)

What was the result of Mendel’s experiment?

•All members of the F1 generation displayed the dominant traits

•Seemed as if one trait had disappeared!

•The members of the F2 generation displayed a 3:1 ratio of dominant to recessive

•The trait reappeared!

The Result of Mendel’s Experiments.

Mendel’s Hypotheses:– For each inherited character, a person has 2

copies of the gene – one from each parent– There are alternative versions of genes

(tall/short, wrinkled/round, etc.)– When 2 alternative versions occur in the same

individual, one is expressed and the other is hidden

Genetic Rules• Rule of Unit Factor:

– There are genes on chromosomes that control traits and at least 2 versions of each trait exists

• Rule of Dominance– One trait is dominant to the other and masks the

other gene when it is present

• Law of Segregation– Traits are inherited independent of each other

• Law of Independent Assortment– Homologous chromosomes sort into the

gametes independently of each other

Some more terms

• Alleles –different versions of a gene

• Dominant – the gene that is always expressed if it is present – capital letter

• Recessive – must have 2 copies to be expressed, otherwise it is hidden by the dominant gene – lower-case letter

• Homozygous – person has 2 of the same allele – TT or BB or tt or bb

• Heterozygous – person has 2 different alleles – Tt or Bb

Genotype• The genes that an

organism possesses• Homozygous• Heterozygous

Phenotype• The way an organism

looks• Only reveals the

genotype if the recessive trait is displayed

How do we determine the genotype and phenotype of the offspring

• Used to determine probability

• Monohybrid

• Dihybrid

• Trihybrid

Punnett Squares!

More Complex Inheritance

• Incomplete Dominance – the heterozygote has a blend of the 2 traits

• Codominance – both traits occur equally

• Multiple Alleles – more than two alleles for a trait

• Polygenic Inheritance – more than one gene controls the trait – usually shows a wide range of variation – skin color and height

Nature vs. NurtureThe effect of the environment

• Environmental effects can determine if a gene is expressed– Chemicals, stress, sunlight, nutrition, etc

• Internal environment can also determine if a gene is expressed– Gender, weight, hormone imbalances

Sex-linked Genes

• On the X or Y Chromosome

• Examples: Color-blindness, hemophilia, duchene’s muscular dystrophy

• Disorders occur more frequently in males

Codominance

Incomplete Dominance

Multiple alleles

Semi Dominance