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Genetics. Review Chromosomes are paired (homologous) in a 2N organism. Each chromosome contains many genes Homologous chromosomes carry the same genes.

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Presentation on theme: "Genetics. Review Chromosomes are paired (homologous) in a 2N organism. Each chromosome contains many genes Homologous chromosomes carry the same genes."— Presentation transcript:

1 Genetics

2 Review Chromosomes are paired (homologous) in a 2N organism. Each chromosome contains many genes Homologous chromosomes carry the same genes. Homologous pair of chromosomes

3 Review Genes can have different versions of the same trait. These versions are called alleles. Homologous chromosomes separate into different gametes. alleles

4 Alleles - Dominant and Recessive Each parent will donate one allele. Alleles can be physically apparent (dominant) or physically hidden (recessive). Mendel worked with traits that had only two alleles of each trait, one dominant and one recessive. MotherFather Alleles

5 Mendels Laws The Law of Segregation –each pair of alleles separates during meiosis (gamete formation) The Law of Dominance –When two alleles differ, –one can control the trait - Dominant –the other can be hidden - Recessive The Law of Independent Assortment –Pairs of genes segregate into gametes randomly and independently (due to random separation of homologous pairs.

6 How Can We Demonstrate these Laws? Mendel recorded phenotypes ( form of the trait that he saw) Based on outcomes from matings (called crosses) over several generations, he could determine the genotypes (the actual genetic makeup for that trait) of previous generations –Homozygous - both alleles for a trait are the same –Heterozygous - alleles are not identical.

7 Monohybrid Crosses Studying the inheritance of a single trait while ignoring all other traits Start with 2 pure parents –P Generation GG (only green) gg (only yellow) –F 1 generation All green heterozygous

8 Determining Genotype (1 Trait) greenyellow 4All green Self- fertilized Self fertilization of this generation produces a different pattern for homozygous vs. heterozygous Cannot see the yellow in this generation Yellow returns PARENT F1F1 F2F2

9 Using a Punnett Square instead Grid for Organizing Genetic Information Shows Probability that a combination will occur 1) Determine Parent alleles 2) Place alleles on edge of square 3) Combine alleles inside the boxes 4) Determine genotypes and phenotypes GGGG g Parent 1 Parent 2 Gg Possible offspring genotypes for F1

10 Heterozygous cross Phenotypes vs. Genotypes –The recessive trait will show ONLY in the absence of the dominant (G) trait. GgGg GgGg GG Gg gggG GG : Gg or gG : gg 1 : 2 : 1 GreenYellow 3 : 1 GENOTYPE PHENOTYPE

11 Test Crosses Purpose - –To distinguish between a homozygous dominant and a heterozygous dominant –Cross the unknown with a known homozygous recessive. –Create two Punnett squares. Cross a homozygous dominant with a homozygous recessive Cross a heterozygous dominant with a homozygous recessive How are the patterns different?

12 Heterozygous + Homozygous Recessive Phenotypes vs. Genotypes –The recessive trait will show ONLY in the absence of the dominant (G) trait. What will happen? g GgGg GG :Gg or gG : g : GreenYellow : GENOTYPE PHENOTYPE GENOTYPE

13 Predicting any single outcome Given Mendels Laws, the probability of an outcome for any one combination can be estimated. Can Use coins to estimate probability –See Exercise on Probability.

14 Probabilities and Proportions Should approximately match. Are only estimates of what SHOULD happen. Can be easily changed If randomness is compromised.

15 Dihybrid crosses Allow the study of two traits at a time. RY ry Make a P cross (RRYY x rryy) Cross F1 plants among themselves (RrYy x RrYy) List all gametes on a 4 x 4 Punnet Square

16 Intermediate Inheritance Mendels study –Dominant and recessive Many genes have intermediate inheritance –Heterozygous phenotype not like either homozygous phenotype

17 Types of Intermediate Inheritance Incomplete Dominance Heterozygote phenotype is inbetween the two homozygous phenotypes –EXAMPLE: Red + White = Pink Flowers BUT: Gametes will contain either Red or White! No dominant phenotype so only capital letters with superscripts are used. Red = F r F r White = F w F w Pink = F r F w

18 Types of Intermediate Inheritance Codominance Both alleles express themselves fully –EXAMPLE: Blood types A, B, AB

19 Polygenic Traits Trait is controlled by two or more gene pairs Eye color, skin, hair –Tone amount, and distribution of pigments are all factors. Each is controlled by a different gene pair, for each attribute (color, skin, hair)

20 Multiple alleles Three or more alleles in a trait Blood type –I A I B and i –Both I A and I B are dominant over I – I A and I A are codominant. –NOT THE SAME as polygenic A single pair of genes with > 2 alleles.

21 Environmental Stress on Genetics Seasonal Changes( external changes) Turning on beard growth (internal changes) Genes can be turned on and off.


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