Introduction to Genetics

Introduction to Genetics Genetics is the study of heredity The inheritance of traits

A trait is a characteristic passed from parent to offspring. Some human traits include:

In the 1800’s Gregor Mendel used pea plants to study the inheritance of traits A trait is a specific characteristic that varies between one individual in a population to another When fertilization takes place it, produces a new cell Pea plants self fertilize so they inherit their own characteristics These are true breeding and produce purebred offspring

Mendel removed the male parts of the pea flowers and made controlled crosses between plants with different traits This is called cross pollination

Cross pollination produces seeds that have two different parents Mendel studied seven traits in the pea plants He chose these traits because they show only one of the two contrasting characteristics

The different forms of a gene that produce traits • Are called alleles • The first cross breeding of different parent plants creates a hybrid filial • Also called F1 generation • Crosses between parents with different traits are called hybrids • When Mendel crossed true breeding tall pea plants with true breeding short plants he created a hybrid

Mendel crossed tall and short pea plants to create a hybrid All of the offspring in the F1 generation were tall because the allele for the tall trait was dominant over the allele for the short trait Did the allele for the recessive trait disappear? When he let the F1 generation self pollinate the recessive trait re-appeared At some point, the recessive allele segregated from the dominant allele

When Mendel repeated other crosses he obtained similar results

Which traits for the pea color and parakeet color are recessive? How do you know?

Mendel realized that the principal of probability can be used to predict the outcome of genetic crosses Mendel concluded that traits are inherited through the passing of factors from parents to offspring Today these chemical factors that determine traits are called genes The principal of dominance states that some alleles are dominant and some are recessive

This chart shows the segregation of alleles during a cross between two hybrid parents The capital letter T represents the dominant allele The lower case letter t represents the recessive allele Punnett squares can be used to predict and compare genetic variations that may result from a cross Shows that the genes for traits sort independently

Phenotype and Genotype • Organisms that have two identical alleles for a trait are called homozygous (TT), or (tt) • These are known as purebreds • (TT) is homozygous dominant • (tt) is homozygous recessive • Organisms that have two different alleles for the same trait are heterozygous (Tt) • These are known as hybrids • An organisms physical characteristics is called the phenotype • The genetic makeup discussed above is called the genotype

A one factor Punnett square (monohybrid cross) follows one gene during a cross

A two factor Punnett square (dihybrid cross) follows two different genes from one generation to the next Mendel found that traits would follow the laws of probability and show independent assortment Found that different traits can segregate independently during fertilization

Intermediate inheritance • Purely dominant and recessive traits are rare in nature • Many genes show an intermediate inheritance • Incomplete dominance is when neither allele dominates over the other • Shows a blending of the two traits

When different dominant traits are crossed and both are present in the offspring this is co-dominance A cross between a black chicken (BB) and a white rooster (WW) X Producing a speckled offspring (BW)

a second type ofco-dominance is human blood type Like type AB blood

When two or more alleles for a trait exist in a population the gene has multiple alleles Fur color in rabbits has multiple alleles Blood type in humans also has multiple alleles

Traits controlled by two or more genes are called polygenetic traits Human skin color is a polygenetic trait, over 4 genes are involved in determining skin color Eye color is also polygenetic

The environment can also have an effect on gene expression Seasonal coat color in foxes and seasonal wing coloration on butterflies Hydrangea flower color to soil pH, goby fish change sex in male dominated populations are all examples of gene expression responding to environmental conditions

Another scientist that tried to advance the study of genetics was Thomas Morgan • He used fruit flies called drosophila melanogaster • They produced large numbers of offspring in a very short time • Only have 8 chromosomes to track • He found that Mendel’s principals also applied to the flies • The basics of Mendelian genetics can also apply to human genetics

The farther apart two genes are on a chromosome, the more likely they are separated by a crossover The frequency of crossing-over between genes can be used to construct genetic maps

Introduction to Genetics