2What happens when you consider more than one gene when you cross 2 parents? Independent assortmentMendel performed more experiments with the pea plants to see what would happen if 2 genes were followed.This is known as a Two-Factor Cross or a Dihybrid cross.
3The Two Factor Cross: F1 Mendel isolated the following pea plants Plants that produced only round yellow peas (genotype RRYY – homozygous dominant)Plants that produced wrinkled green peas (genotype rryy – homozygous recessive)Results – all of the F1 offspring produced round yellow peas.
7100% of the offspring were Round and Yellow. Where did the wrinkled green trait go?This did not show whether genes will assort independentlyIt did however set up heterozygous offspring (RrYy)All offspring are heterozygous dominant for Round yellow seeds.
8The Two factor cross: F2Mendel knew that all plants were heterozygous dominant for Round, yellow peas (RrYy)How would the genes segregate when the F1 generation was crossed?Remember that the F1 generation was formed by the fusion of gametes carrying the dominant RY alleles and the other carrying the recessive ry alleles.
9Does this mean that the dominant alleles always stay together? Or would they segregate independently?556 peas were produced in the F2 generation
10Mendal’s data: 556 Total Peas in F2 generation PhenotypeNumber of PeasYellow and Round (dominant traits)315Wrinkled and Green (recessive traits)32Other combinations - Yellow and wrinkled/Green and Round209
12Figure 11-10 Independent Assortment in Peas Section 11-3
13Notice the other combinations These phenotypes –Yellow and wrinkledGreen and roundWERE NOT SEEN IN EITHER PARENTWhat does this mean?
14This meant that the alleles for seed shape segregated independently of those for seed color. This is known as Independent Assortment.Independent assortment – independent segregation of genes during the formation of gametes.
15Independent assortment explains why traits such as nose shape and eye color do not always go together.The shape of your dad’s nose and his eye color will segregate separately.This is why you can have dad’s nose and mom’s eyes
17Ability? Can you understand the language of genetics? Read the top of page 272 “A SUMMARY OF MENDEL’S PRINCIPLES.Can you read this and understand it?
18The inheritance of biological characteristics is determined by individual units known as genes. Genes are passed from parents to their offspring.In cases in which two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.In most sexually reproducing organisms, each adult has two copies of each gene—one from each parent. These genes are segregated from each other when gametes are formed.The alleles for different genes usually segregate independently of one another.
19Beyond Dominant and Recessive Alleles Mendel’s contribution to genetics was great, but there is more to the story – most of which is not covered in this course.Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.
20Incomplete dominanceThere is a plant named “four o’clock” (genus Mirabilis)Mirabilis can have flowers of 3 colors – Red, white, and pink.If you cross Red flowers with white flowers, you get pink flowers.
21Figure 11-11 Incomplete Dominance in Four O’Clock Flowers Section 11-3
25Codominance Some chickens demonstrate codominance. This next example is not true of all chickens.If you cross a black chicken and a white chicken, the offspring will be speckled with black and white feathers.
28Multiple alleles Many genes have more than 2 alleles. These are said to have multiple alleles.This does not mean that one individual will have more than 2 alleles in the genotype, it simply means that there are many alleles in the population
29Rabbit hair color is a good example of multiple alleles. There is one gene for hair color and there are four different alleles.
34Polygenic Traits Some traits are created as a result of many genes. These are called polygenic traits.It is believed that human skin color is controlled by 4 genes.
35Genetics and the environment Genes don’t control everything.A plant can a have a “tall” gene, but if it does not receive enough sunlight, it will not be tall.A human can have an “obese” gene, but if he does not have enough nourishment, he will not be obese.