Presentation on theme: "Question Set 3:Mendel’s Law of Independent"— Presentation transcript:
1Question Set 3:Mendel’s Law of Independent What is the law of Independent assortment?What evidence supports the law of independent assortment?What is a dihybrid cross?How many types of gametes can a parent that is a heterozygote for 2 traits make for those traits?Why don’t alleles that exist on a homolog all get inherited together every time?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 nucleusThus, gene pairs on one chromosome get sorted into gametes independently of gene pairs on other chromosomesPunnett squares can be used to predict inheritance patterns of two or more genes simultaneously
3Dihybrid CrossIn a dihybrid cross, individuals identically heterozygous for alleles of two genes (dihybrids) are crossed, and the traits of the offspring are observeddihybrid crossBreeding experiment in which individuals identically heterozygous for two genes are crossedThe frequency of traits among the offspring offers information about the dominance relationships between the paired alleles
4A Dihybrid CrossStart 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 F1 offspring will be dihybrids (PpTt) and have purple flowers and tall stems
5A Dihybrid Cross (cont.) Then cross two F1 plants: a dihybrid cross (PpTt X PpTt)Four types of gametes can combine in sixteen possible waysIn F2 plants, four phenotypes result in a ratio of 9:3:3:19 tall with purple flowers3 short with purple flowers3 tall with white flowers1 short with white flowers
6Law 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 ratioEach trait (gene pair) sorted into gametes independently of other traits (gene pairs)law of independent assortmentDuring meiosis, members of a pair of genes on homologous chromosomes get distributed into gametes independently of other gene pairs
7Independent Assortment 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.ormeiosis Imeiosis IC 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.Figure 13.7 Independent assortment.meiosis IImeiosis IID Thus, when sister chromatids separate during meiosis II, the gametes that result have one of four possible combinationsof maternal and paternal chromosomes.gamete genotype:ptPTpTPtFig. 13.7, p. 194
8A Dihybrid CrossFigure A dihybrid cross between plants that differ in flower color and plant height. P and p stand for dominant and recessive alleles for flower color. T and t stand for dominant and recessive alleles for height.Fig , p. 195
9A Dihybrid CrossFigure A dihybrid cross between plants that differ in flower color and plant height. P and p stand for dominant and recessive alleles for flower color. T and t stand for dominant and recessive alleles for height.Fig , p. 195
10The Contribution of Crossovers Genes that are far apart on a chromosome tend to assort into gametes independently because crossing over occurs between them very frequentlyGenes that are very close together on a chromosome are linked, they do not assort independently because crossing over rarely happens between them
11Key Concepts Insights From Dihybrid Crosses Pairs of genes on different chromosomes are typically distributed into gametes independently of how other gene pairs are distributedBreeding experiments with alternative forms of two unrelated traits can be used as evidence of such independent assortment
12Question Set 3:Mendel’s Law of Independent Assortment What is the law of Independent assortment?What evidence supports the law of independent assortment?What is a dihybrid cross?How many types of gametes can a parent that is a heterozygote for 2 traits make for those traits?Why don’t alleles that exist on a homolog all get inherited together every time?Are there some alleles that are often inherited together? Why?
13Questions Set 4: Patterns of Inheritance What genotype do you have to have to express a recessive phenotype?What inheritance patterns does (ABO) blood type follow there are 2 ?What patter does color in snapdragons follow and how many phenotypes are there for this trait?Describe an example of epistasis in dog coat color.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?The continuous variation of eight in humans occurs because of two factors what are they?
1413.5 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 oneOther patterns of inheritance are not so simple:CodominanceIncomplete dominanceEpistasisPleiotropy
15CodominanceCodominant alleles are both expressed at the same time in heterozygotes, as in multiple allele systems such as the one underlying ABO blood typingcodominantRefers to two alleles that are both fully expressed in heterozygous individualsmultiple allele systemGene for which three or more alleles persist in a population
16Codominance: ABO Blood Types Which two of the three alleles of the ABO gene you have determines your blood typeThe A and the B allele are codominant when pairedGenotype AB = blood type ABThe O allele is recessive when paired with either A or BGenotype AA or AO = blood type AGenotype BB or BO= type BGenotype OO = type O
18Codominance: ABO Blood Types AA or AOBB or BOGenotypes:ABOOFigure Combinations of alleles that are the basis of human blood type.Phenotypes (blood type):AABBOFig. 13.9, p. 196
19Incomplete DominanceWith incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypesincomplete dominanceCondition in which one allele is not fully dominant over another, so the heterozygous phenotype is between the two homozygous phenotypes
20Incomplete Dominance: Snapdragons In snapdragons, one allele (R) encodes an enzyme that makes a red pigment, and allele (r) makes no pigmentRR = red; Rr = pink; rr = whiteA 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
21Incomplete Dominance: Snapdragons 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.Figure Incomplete dominance in heterozygous (pink) snapdragons. An allele that affects red pigment is paired with a ‘white’ allele.B If two of the pink heterozygotesare crossed, the phenotypesof the resulting offspring will occur in a 1:2:1 ratio.Fig , p. 196
22EpistasisSome traits are affected by multiple gene products, an effect called polygenic inheritance or epistasisepistasisEffect in which a trait is influenced by the products of multiple genes
23Epistasis: Labrador Retriever Labrador retriever coat color, can be black, brown, or yellowFigure Epistasis in dogs. Epistatic interactions among products of two gene pairs affect coat color in Laborador retrievers. Left, all dogs with an E and B allele have black fur. Those with an E and two recessive b alleles have brown fur. All dogs homozygous for the recessive e allele have yellow fur. Right: black, chocolate, and yellow Laborador retrievers.
24Epistasis: Labrador Retriever A dominant allele (B) specifies black fur, and its recessive partner (b) specifies brown furA dominant allele of a different gene (E ) causes color to be deposited in fur, and its recessive partner (e) reduces colorA dog with an E and a B allele has black furA dog with an E allele and homozygous for b is brownA dog homozygous for the e allele has yellow fur regardless of its B or b alleles
25Epistasis: Labrador Retriever Figure Epistasis in dogs. Epistatic interactions among products of two gene pairs affect coat color in Laborador retrievers. Left, all dogs with an E and B allele have black fur. Those with an E and two recessive b alleles have brown fur. All dogs homozygous for the recessive e allele have yellow fur. Right: black, chocolate, and yellow Laborador retrievers.
26Pleiotropy 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 syndromepleiotropicRefers to a gene whose product influences multiple traits
27Pleiotropy: Marfan Syndrome In Marfan syndrome, mutations affect elasticity of tissues of the heart, skin, blood vessels, tendons, and other body partsHaris Charalambous died when his aorta burst – he was 21
2813.6 Complex Variation in Traits Phenotype often results from complex interactions among gene products and the environmentMany traits show a continuous range of variation
29Continuous VariationSome 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 variationcontinuous variationIn a population, a range of small differences in a shared trait
30Continuous 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 continuouslybell curveBell-shaped curveTypically results from graphing frequency versus distribution for a trait that varies continuously
31Continuous Variation (Cont.) Human height and eye color are traits that vary continuously
32Continuous Variation (Cont.) Figure Continuous variation in height among male biology students at the University of Florida. The students were divided into categories of one-inch increments in height and counted (bottom). A graph of the resulting data produces a bell-shaped curve (top), an indication that height varies continuously.
33Environmental Effects on Phenotype Environmental factors often affect gene expression, which in turn affects phenotype:Seasonal change in animal fur colorsSpines grow in presence of predatorsDifferent plant heights when grown at different altitudes
34Environmental Effects on Phenotype In summer, the snowshoe hare’s fur is brown; in winter, white – offering seasonal camouflage from predatorsFigure Example of environmental effects on animal phenotype. The color of the snowshoe hare’s fur varies by season. In summer, the fur is brown (left); in winter, white (right). Both forms offer seasonally appropriate camouflage from predators.
35Key Concepts Variations on Mendel’s Theme Not all traits appear in Mendelian inheritance patternsAn allele may be partly dominant over a nonidentical partner, or codominant with itMultiple genes may influence a trait; some genes influence many traitsThe environment also influences gene expression
36Questions Set 4: Patterns of Inheritance What genotype do you have to have to express a recessive phenotype?What inheritance patterns does (ABO) blood type follow there are 2 ?What patter does color in snapdragons follow and how many phenotypes are there for this trait?Describe an example of epistasis in dog coat color.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?The continuous variation of hieght in humans occurs because of two factors what are they?