Introduction to Genetics

Introduction to Genetics

Dr. Donna HowellBlacksburg High School

2013-2014

What is Genetics?• Have you ever

wondered how traits are passed from parents to the children?

• Genetics is the study of heredity, or how traits are passed down from generation to generation.

Gregor Mendel• Gregor Mendel

was a monk who lived in the 1800’s, and he is famous for the work he did with pea plants in the field of genetics.

Gregor Mendel• He knew that the

eggs of the female plants and the sperm of the male plants joined together in the process of fertilization.

Gregor Mendel• Mendel

studied 7 traits of the pea plants: – Seed shape– Seed color– Seed coat color– Pod shape– Pod color– Flower position– Plant height

Gregor MendelTrait

Alleles

Findings of cross

Genes and Dominance• The offspring

crosses between parents with different traits, like on the previous slide, are called “hybrids.”

• He concluded that:– Inheritance is

determined by factors that determine traits, called genes.

– The different forms of a gene are called alleles.

Dominant and Recessive

– Mendor concluded that some genes are dominant, meaning they show up in the offspring, and some are recessive, meaning they do not show up in the offspring.

– Example: For seed shape, round (R) is dominant, and wrinkled (r) is recessive.

– If you cross a R with an r, the seed shape is round.

Segregation– Mendel now wanted

to know if, since a Rr seed shape is round, what happened to the recessive r gene?

– This time, he crossed Rr with Rr plants, and observed what happened.

– He discovered that some of the recessive traits reappeared!

– He concluded that the R gene was separate, or segregated, from the r gene.

Segregation– Mendel

concluded that the R and r gene became segregated during the formation of sex cells, or gametes.

– What this means is that each gamete only carries a single copy of each gene (either R or r)

Probability– Mendel

realized we could use probability to predict what the gene types of crosses between male and female will be.

– We do this using a Punnett square.

Punnett Squares– A Punnett square

is a diagram that shows possible outcoming of mating.

Vocabulary Words– If you have two of

the same type of allele together, such as RR or rr, they are called homozygous.

– If you have two different alleles together, such as Rr, they are called heterozygous.

Vocabulary Words– When talking

about the outcome of crosses, two other words are used:• Genotype: what

the genes of an organism look like – Rr or rr or RR

• Phenotype: what you actually see when looking at the organism – round or wrinkled peas.

Monohybrid Cross– Let’s do a simple

monohybrid cross Punnett square:

R R

r Rr(round)

Rr(round)

r Rr(round)

Rr(round)

Genotype: 100% Rr

Phenotype: 100% round

Monohybrid Cross– Let’s see what

happens when we cross two of the Rr offspring:

R r

R RR(round)

Rr(round)

r Rr(round)

Rr(wrinkle

d)

Genotype: 50% Rr, 25% RR and 25% rr

Phenotype: 75% round, 25% wrinkled

Independent Assortment

– Mendel found that the alleles for seed shape segregated independently of those for seed color (they were not attached)

– This happens during the formation of gametes

Dihybrid Cross

Other Types of Dominance

• So far we have only looked at alleles that have a dominant and recessive allele.

• But there are other types of dominance:– Incomplete

dominance– Codominance– Multiple alleles– Polygenic traits

Incomplete Dominance– When neither allele

is completely dominant over the other.

– The offspring are a mixture of each allele.

Codominance– In this case, BOTH

alleles contribute to the phenotye.

– Both alleles are expressed.

Multiple Alleles– Many genes have

more than two alleles for a trait.

– Example: coat color in rabbits.

Polygenic Traits– Traits that are

controlled by two or more genes.

– Example: human skin color

Meiosis– Meiosis is the

process of reduction division in which the number of chromosomes per cell is cut in half through the separation of like chromosomes in a cell.

– Happens in sperm and egg cells

– Why might the number of chromosomes need to be cut in half?

Meiosis I

Meiosis II

Mitosis vs. Meiosis

The End!