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Beyond Mendel

Beyond Mendel. Dominance. Alleles are not termed dominant because they beat down the recessive allele. Alleles are just variations in each genes nucleotide code. In a heterozygous genotype, the two alleles coexist but do not affect each other.

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Beyond Mendel

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  1. Beyond Mendel

  2. Dominance • Alleles are not termed dominant because they beat down the recessive allele. • Alleles are just variations in each genes nucleotide code. • In a heterozygous genotype, the two alleles coexist but do not affect each other. • Dominant alleles are not necessarily more common than recessive either. • The allele for 5 fingers is recessive and more common that the allele for 6 fingers (polydactyly). 99.7% of people are recessive for this allele. Complete Dominance Intermediate Dominance Codominance

  3. Lack of Dominance • Mendel's F1 offspring always resembled the dominant parent, because each of the genes Mendel chose to study showed complete dominance. • When there is no dominant allele, the heterozygote will have a phenotype different from either homozygous form. • This is sometimes referred to as an intermediate phenotype. • There are a number of variations in lack of dominance, but each results in heterozygous conditions that have a phenotype different from either homozygous phenotype. • In other words, when a gene lacks dominance, there will be three different phenotypes, two homozygous phenotypes (AA and aa) and a third heterozygous phenotype (Aa).

  4. Beyond Mendel • Mendel did great work, but he did not know much about chromosomes, molecular genetics or meiosis. • Mendel's genes had dominant and recessive forms, and that each inheritable trait was found on different chromosomes. • Not all inheritance results match Mendel's predictions. • Many phenotypic expressions are the result of gene interactions. • Each gene involved may fit a Mendelian prediction, but their method of interaction alters the predicted ratios.Some gene actions go beyond the basic Mendelian predictions.

  5. Incomplete Dominance • The F1 generation always looked like one of the parents in Mendel’s experiments since there was complete dominance of one allele over another. • For some genes, the F1 hybrids show an intermediate between the two parental phenotypes. This is called incomplete dominance. • Snapdragons: • When CRCR (Red) is crossed with CWCW (white), all of the hybrids (F1) have pink flowers. • This arises because the heterozygotes (CRCW) contain less red pigment than their red homozygotes. • This is NOT blending inheritance. Why? Because we can isolate out the original traits of red and white by crossing the F1 generation. CR CW CR CW 1:2:1 ratio

  6. Complete and Codominant Alleles • Complete dominance refers to the phenotypes of the heterozygote (Pp) and homozygote (PP) being indistinguishable (both purple). • On the other end of the spectrum is co-dominance. • Both alleles are equally expressed in the heterozygote - both appear in the heterozygous individuals • Blood Groups M,N and MN • The groupings are based on the presence of 2 molecules attached to the RBC • M people have one of the two, N people have the other, and MN have both. • Why? Well, a single gene locus, at which 2 variations are possible, determines these blood groups: M (homozygous for 1 allele), N (homozygous for the other allele), and MN (heterozygous) • The MN is NOT an intermediate because both the M and N are expressed in the phenotype.

  7. The MN blood group system is under the control of an autosomal locus found on chromosome 4, with two alleles designated LM and LN. The bloodtype is due to a glycoprotein present on the surface of red blood cells, which behaves as a native antigen. Phenotypic expression at this locus is codominant because an individual may exhibit either one or both antigenic substances. Frequencies of the two alleles vary widely among human populations

  8. Multiple Alleles • Most genes contain more than 2 allele forms. • Blood Type Example • You can either be A,B,AB or O blood type. • Each is determined by the presence/absence of 2 carbohydrates found on the RBC. • A person who is A has carbohydrate A, B-carbohydrate B, AB both carbs, and O has neither. • These blood groups arise from different alleles: IA, IB, and i. • IA and IB are dominant to i. • So, IA IA and IAi people are A type, IB IB and IBi are B types and ii are O type since neither A or B substance are produced. • IA IB are co-dominant giving rise to AB blood phenotype. • This is why some folks cannot donate A or B because that substance is foreign but O type has neither.

  9. ABO Blood Groups

  10. ABO Blood Group • A A or AB, B B or AB, AB AB, O  All • 40% O, 40% A, 15% B, and 5% AB (Europeans) • Chromosome 9, 1062 letters long • 6 short and long exons (paragraphs) of the chromosome • Difference btw A and B is only 7 letters out of 1062 • C,G,C,G (letters 523, 700, 793 and 800) A-type, B-type read G,A,A,C • O groups folks only have 1 letter difference but it is missing the 285th letter G.

  11. Epistasis (means standing upon) • One gene pair controls or alters the expression of other gene pairs, so that expected phenotypes do not appear. • Usually involves pigments. • The gene to distribute pigment can be overridden by a second gene which blocks (inhibits) or alters pigment production. • In Labrador dogs, • the gene "B" produces dark pigment. • The gene "E" controls the distribution of the pigment in fur. • A dog with eebb will have yellow coat color and brown nose and lips because it produces some pigment but can't distribute the pigment in fur. • A dog with eeB_ will have yellow coat color but a black nose and lips; it can produce the dark pigment but can't distribute it in fur. • A chocolate lab will be E_bb. It produces some pigment and can distribute the pigment (no E inhibition). • A black lab will be E_B_, and can both produce dark pigment and distribute the dark pigment in its fur.

  12. Epistasis

  13. eebb eeB_ E_bb E_B_

  14. Epistasis in Mice • Mice also have black or brown pigmented fur depending on the inheritance of a gene for pigmentation. • A second, independent gene (cc) prevents the distribution of any pigment in the fur. This gene, when recessive, results in white mice. 9:3:4 ratio

  15. Human Eye Color • Inheritance of Eye Color in Humans - a special case of two genes that lack dominance, both of which produce pigment • We have an increase in variation within the population because the heterozygotes of the genes involved are also expressed. • The eye color genes code for the production of a yellow-brown pigment

  16. First Iris Layer PigmentAA = Produce lots of pigmentAa = Produce some pigmentaa = Do not produce pigmentSecond Iris Layer PigmentBB = Produce lots of pigmentBb = Produce some pigmentbb = Do not produce pigment

  17. Polygenic Inheritance • Gene Interactions with multiple genes or an additive effect of two or more genes on a single phenotype character. • The individual phenotype is the result of the combined interaction of all the alleles at all of the gene loci involved. • When a number of genes interact resulting in a number of different phenotypes, we see continuous variation in the population, with respect to that genetic characteristic. • Continuous variation involves several gene pairs on independent chromosomes, all of which specify additive information for the same trait. • Continuous variation can most easily be demonstrated when population data shows a bell-shaped distribution pattern when graphed. • Because of this, polygenic inheritance is sometimes called Quantitative Inheritance. • Skin and hair pigmentation and height are two examples of polygenic inheritance in humans. It is believed that there are at least 3 independent genes, each of which lacks dominance, responsible for producing the melanin pigment in human skin (and in hair). • Note that production of pigment is also activated by environmental triggers, but only to the extent that the genes can code for.

  18. Human Inheritance • Pedigree: as much information about a specific trait is collected through family history. • Widows Peak: • Dominant allele W • Lack of widow’s peak is a recessive allele (ww) • If grandma and grandpa were heterozygous (Ww) or homozygous (WW) then all offspring will have widow’s peaks. • The F1 generation who do have widow’s peaks must also be heterozygous (Wwxww products). • The F2 generation of this lineage has 2 daughters and the one who has a widow’s peak could be either WW or Ww. • Probability of having a widow’s peak if the intial cross was WwXWw? • Ear Lobes: • F is the dominant allele that results in free earlobes and f for attached earlobes. Thus attached would yield ff and free, Ff or FF. Probability of attached is ¼. • What is the probability of having a widow’s peak and attached earlobes? • (WwFf X WwFf): ¾ (widows peak) x ¼ (attached earlobes) = 3/16

  19. Sex Chromosomes • In mammals there are 2 varieties of sex chromosomes (X and Y). • XX = female, XY = male • Males develop from zygotes that contain one X, one Y and when meiosis occurs in testis, X and Y behave like homologous chromosomes. • In males, half the sperm cells produce X chromosome and half Y. In females, each ovum has one X. • So, if sperm (X) fertilizes and egg (X) then = XX; Sperm (Y) fertilizes egg (X) then XY (male) • Sex is a matter of chance (50/50) • Haplodiploidy in Ants is much different • No sex chromosomes • Females are diploid • Males develop haploid from unfertilized eggs (no dads) • Struggle in the kingdom

  20. Sex Linked Genes and Inheritance • Many genes on the sex chromosomes have other characters unrelated to determining sex. • Sex linked refers to genes usually located on the X chromosome. • Fathers pass sex-linked alleles to all daughters but to no sons. • Mother pass to both. • Of course if the sex-linked trait is recessive allele, then only the homozygous female will express the phenotype. • Males will express the allele in any recessive form. • Because of this, more males than females have disorders passed as recessive sex-linked recessives. • There re exceptions: • Color blindness may occur in a daughter whose dad was color blind and whose mom was a carrier. • This is rare.

  21. Recessive Trait Transmission a. Father with the trait transmits the mutant allele to all daughters but no sons. Daughters will only be carriers (normal phenotype) when the mom is a dominant homozygote. b. A carrier who mates with a normal male will pass the mutation to ½ of her sons and ½ daughters. The sons with the mutation will have the disorder, the daughters who only have a single dose of the gene will be carriers but not have the disorder. c. If a carrier mates with a male who has the trait, there is a 50% chance that each child born will have the trait regardless of sex. Daughters who do not have the trait will be carriers, but males without the trait will be completely free of harm.

  22. Sex Linked Diseases • Muscular Dystrophy • Affects about 3500 kids who rarely live past 20. • Absence of a muscle protein causes progressive weakening of muscles. This gene is located on the X chromosome. • Hemophilia • Sex-linked recessive trait that’s phenotype is the absence of a blood clotting protein. • Women are carriers and sons usually get the disorder. • Color Blindness (red/green)

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