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Introduction to Genetics. Dr. Donna Howell Blacksburg 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.
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Introduction to Genetics Dr. Donna Howell Blacksburg 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 Mendel Trait 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 Rrwith 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: Genotype: 100% Rr Phenotype: 100% round
Monohybrid Cross • Let’s see what happens when we cross two of the Rr offspring: 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
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?
