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Question Set 3:Mendel’s Law of Independent 1.What is the law of Independent assortment? 2.What evidence supports the law of independent assortment? 3.What.

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Presentation on theme: "Question Set 3:Mendel’s Law of Independent 1.What is the law of Independent assortment? 2.What evidence supports the law of independent assortment? 3.What."— Presentation transcript:

1 Question Set 3:Mendel’s Law of Independent 1.What is the law of Independent assortment? 2.What evidence supports the law of independent assortment? 3.What is a dihybrid cross? 4.How many types of gametes can a parent that is a heterozygote for 2 traits make for those traits? 5.Why don’t alleles that exist on a homolog all get inherited together every time? 6.Are there some alleles that are often inherited together? Why?

2 \Mendel’s Theory of Independent Assortment When homologous chromosomes separate during meiosis, either one of the pair can end up in a particular nucleus Thus, gene pairs on one chromosome get sorted into gametes independently of gene pairs on other chromosomes Punnett squares can be used to predict inheritance patterns of two or more genes simultaneously

3 Dihybrid Cross In a dihybrid cross, individuals identically heterozygous for alleles of two genes (dihybrids) are crossed, and the traits of the offspring are observed dihybrid cross Breeding experiment in which individuals identically heterozygous for two genes are crossed The frequency of traits among the offspring offers information about the dominance relationships between the paired alleles

4 A Dihybrid Cross Start with one parent plant that breeds true for purple flowers and tall stems (PPTT ) and one that breeds true for white flowers and short stems (pptt) Each plant makes only one type of gamete (PT or pt) All F 1 offspring will be dihybrids (PpTt) and have purple flowers and tall stems

5 A Dihybrid Cross (cont.) Then cross two F 1 plants: a dihybrid cross (PpTt X PpTt) Four types of gametes can combine in sixteen possible ways In F 2 plants, four phenotypes result in a ratio of 9:3:3:1 9 tall with purple flowers 3 short with purple flowers 3 tall with white flowers 1 short with white flowers

6 Law of Independent Assortment Mendel discovered the 9:3:3:1 ratio in his dihybrid experiments – and noted that each trait still kept its individual 3:1 ratio Each trait (gene pair) sorted into gametes independently of other traits (gene pairs) law of independent assortment During meiosis, members of a pair of genes on homologous chromosomes get distributed into gametes independently of other gene pairs

7 Fig. 13.7, p. 194 meiosis II meiosis I A This example shows just two pairs of homologous chromosomes in the nucleus of a diploid (2n) reproductive cell. Maternal and paternal chromosomes, shown in pink and blue, have already been duplicated. B Either chromosome of a pair may get attached to either spindle pole during meiosis I. With two pairs of homologous chromosomes, there are two different ways that the maternal and paternal chromosomes can get attached to opposite spindle poles. C Two nuclei form with each scenario, so there are a total of four possible combinations of parental chromosomes in the nuclei that form after meiosis I. D Thus, when sister chromatids separate during meiosis II, the gametes that result have one of four possible combinations of maternal and paternal chromosomes. gamete genotype: meiosis II meiosis I or PtpTPTpt Independent Assortment

8 Fig , p. 195 A Dihybrid Cross

9 Fig , p. 195 A Dihybrid Cross

10 The Contribution of Crossovers Genes that are far apart on a chromosome tend to assort into gametes independently because crossing over occurs between them very frequently Genes that are very close together on a chromosome are linked, they do not assort independently because crossing over rarely happens between them

11 Key Concepts Insights From Dihybrid Crosses Pairs of genes on different chromosomes are typically distributed into gametes independently of how other gene pairs are distributed Breeding experiments with alternative forms of two unrelated traits can be used as evidence of such independent assortment

12 Question Set 3:Mendel’s Law of Independent Assortment 1.What is the law of Independent assortment? 2.What evidence supports the law of independent assortment? 3.What is a dihybrid cross? 4.How many types of gametes can a parent that is a heterozygote for 2 traits make for those traits? 5.Why don’t alleles that exist on a homolog all get inherited together every time? 6.Are there some alleles that are often inherited together? Why?

13 Questions Set 4: Patterns of Inheritance 1.What genotype do you have to have to express a recessive phenotype? 2.What inheritance patterns does (ABO) blood type follow there are 2 ? 3.What patter does color in snapdragons follow and how many phenotypes are there for this trait? 4.Describe an example of epistasis in dog coat color. 5.Marfan’s syndrome is a defect in how human’s make their connective tissue fibers. What type of effects does it have on the human body and why? 6.The continuous variation of eight in humans occurs because of two factors what are they?

14 13.5 Beyond Simple Dominance Mendel studied inheritance patterns that are examples of simple dominance, in which a dominant allele fully masks the expression of a recessive one Other patterns of inheritance are not so simple: Codominance Incomplete dominance Epistasis Pleiotropy

15 Codominance Codominant alleles are both expressed at the same time in heterozygotes, as in multiple allele systems such as the one underlying ABO blood typing codominant Refers to two alleles that are both fully expressed in heterozygous individuals multiple allele system Gene for which three or more alleles persist in a population

16 Codominance: ABO Blood Types Which two of the three alleles of the ABO gene you have determines your blood type The A and the B allele are codominant when paired Genotype AB = blood type AB The O allele is recessive when paired with either A or B Genotype AA or AO = blood type A Genotype BB or BO= type B Genotype OO = type O

17 Codominance: ABO Blood Types

18 Fig. 13.9, p. 196 Phenotypes (blood type): Genotypes: O OO BABA AA or AO AB BB or BO Codominance: ABO Blood Types

19 Incomplete Dominance With incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes incomplete dominance Condition in which one allele is not fully dominant over another, so the heterozygous phenotype is between the two homozygous phenotypes

20 Incomplete Dominance: Snapdragons In snapdragons, one allele (R) encodes an enzyme that makes a red pigment, and allele (r) makes no pigment RR = red; Rr = pink; rr = white A cross between red and white (RR X rr) yields pink (Rr) A cross between two pink (Rr X Rr) yields red, pink, and white in a 1:2:1 ratio

21 Fig , p. 196 homozygous (RR) x homozygous (rr) heterozygous (Rr) A Cross a red-flowered with a white-flowered plant, and all of the offspring will be pink heterozygotes. B If two of the pink heterozygotes are crossed, the phenotypes of the resulting offspring will occur in a 1:2:1 ratio. Incomplete Dominance: Snapdragons

22 Some traits are affected by multiple gene products, an effect called polygenic inheritance or epistasis epistasis Effect in which a trait is influenced by the products of multiple genes Epistasis

23 Epistasis: Labrador Retriever Labrador retriever coat color, can be black, brown, or yellow

24 Epistasis: Labrador Retriever A dominant allele (B) specifies black fur, and its recessive partner (b) specifies brown fur A dominant allele of a different gene (E ) causes color to be deposited in fur, and its recessive partner (e) reduces color A dog with an E and a B allele has black fur A dog with an E allele and homozygous for b is brown A dog homozygous for the e allele has yellow fur regardless of its B or b alleles

25 Epistasis: Labrador Retriever

26 Pleiotropy A pleiotropic gene influences multiple traits Mutations in pleiotropic genes are associated with complex genetic disorders such as sickle-cell anemia, cystic fibrosis, and Marfan syndrome pleiotropic Refers to a gene whose product influences multiple traits

27 Pleiotropy: Marfan Syndrome In Marfan syndrome, mutations affect elasticity of tissues of the heart, skin, blood vessels, tendons, and other body parts Haris Charalambous died when his aorta burst – he was 21

28 13.6 Complex Variation in Traits Phenotype often results from complex interactions among gene products and the environment Many traits show a continuous range of variation

29 Continuous Variation Some traits appear in two or three forms; others occur in a range of small differences (continuous variation) The more genes and environmental factors that influence a trait, the more continuous is its variation continuous variation In a population, a range of small differences in a shared trait

30 Continuous Variation (Cont.) If a graph line drawn around the top of the bars showing the distribution of values for a trait is bell-shaped (a bell curve) the trait varies continuously bell curve Bell-shaped curve Typically results from graphing frequency versus distribution for a trait that varies continuously

31 Continuous Variation (Cont.) Human height and eye color are traits that vary continuously

32 Continuous Variation (Cont.)

33 Environmental Effects on Phenotype Environmental factors often affect gene expression, which in turn affects phenotype: Seasonal change in animal fur colors Spines grow in presence of predators Different plant heights when grown at different altitudes

34 Environmental Effects on Phenotype In summer, the snowshoe hare’s fur is brown; in winter, white – offering seasonal camouflage from predators

35 Key Concepts Variations on Mendel’s Theme Not all traits appear in Mendelian inheritance patterns An allele may be partly dominant over a nonidentical partner, or codominant with it Multiple genes may influence a trait; some genes influence many traits The environment also influences gene expression

36 Questions Set 4: Patterns of Inheritance 1.What genotype do you have to have to express a recessive phenotype? 2.What inheritance patterns does (ABO) blood type follow there are 2 ? 3.What patter does color in snapdragons follow and how many phenotypes are there for this trait? 4.Describe an example of epistasis in dog coat color. 5.Marfan’s syndrome is a defect in how human’s make their connective tissue fibers. What type of effects does it have on the human body and why? 6.The continuous variation of hieght in humans occurs because of two factors what are they?


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