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Exploring Mendelian Genetics

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1 Exploring Mendelian Genetics
Introduction to Genetics Chapter 11 Section 3 Honors Biology


3 Independent Assortment
The two-factor cross F1: Mendel bred true bred round yellow (RRYY) with true bred wrinkled green (rryy) All offspring were round yellow (RrYy) F2: Mendel self pollinated the F1 generation to yield the F2 generation The offspring produced a 9:3:3:1 ratio 9 round yellow 3 round green 3 wrinkled yellow and 1 wrinkled green

4 The two factor F2 cross

5 Independent Assortment
Since the F2 generation contained combinations of alleles not found in P generation, this meant that the alleles separated independently of one another. This principal is known as independent assortment – chromosomes segregate independently of one another. Each chromosome is a group of genes The principal of independent assortment states that genes for different traits can segregate independently during the formation of gametes. Independent assortment helps account for the many genetic variations observed in plants animals and other organisms

6 Summary of Mendel's Principals
Biological characteristics are determined by genes In cases in which alleles for genes exist, one allele is dominant and one is recessive In sexual reproducing organisms, there are 2 copies for every gene, one from each parent. The genes are segregated during formation of gametes The allele for different genes usually segregate independently of one another

7 Other Situations Beyond Dominant and Recessive
Some alleles are neither dominant nor recessive, and are controlled by multiple alleles or multiple genes The case in which one allele is not completely dominant over another is called incomplete dominance. The heterozygote phenotype is in between the 2 homozygous phenotypes Ex: A red flower (RR) crossed with a white flower (WW) creates pink offspring (RW)

8 Other Situations Beyond Dominant and Recessive

9 Other Situations Beyond Dominant and Recessive

10 Other Situations Beyond Dominant and Recessive
When both alleles contribute to the phenotype of the offspring this is called codominance Ex: In certain chickens, the allele for black feathers is codominant with white feathers. Heterozygous chicken offspring are both black and white speckled (called erminette) Unlike the blending in incomplete dominance, both alleles appear separately Ex: this can also been seen in humans with cholesterol levels. People with the heterozygous form of the gene produce 2 different forms of the proteins, each with a differnet effect on the cholesterol

11 Other Situations Beyond Dominant and Recessive
Some genes are controlled by more than 2 alleles and are called multiple alleles This does not mean that a person can have more than 2 alleles, it means that more than 2 alleles exist in the population for a single gene Ex: rabbit fur color is controlled by 4 alleles Ex: human blood type (A B or O)

12 Multiple Alleles for Rabbit Color
Full Color: Brown C dominant to all other alleles Chinchilla: Grey Cch partial defect in pigmentation. Dominant to Ch and c alleles Himalayan: Color in Certain parts Ch Dominant to c allele Albino: No color c recessive to all other alleles Example: Blood Type is controlled by 3 alleles A and B are codominant. Both are dominant over O

13 Other Situations Beyond Dominant and Recessive
Some traits are controlled by more than one gene. These are called polygenic traits, which means “having many genes” Polygenic traits often show a wide range of phenotypes Ex: at least 3 genes control the reddish-brown pigment in the eye color of fruit flies Ex: the wide range of skin color is because more than four different genes control this trait

14 Genes and Environment Environment determines your phenotypes as well
Characteristics are controlled by an interaction between genes and environment Ex: a sunflower. Genes may control height and color however some characteristics are also controlled by soil type, water availability, amount of sunlight, and climate Genes provide a plan for development, but the environment depends on how that plan unfolds

15 Class work Chapter 11 Section 3 Assessment

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