mendelian genetics review! genetic terms: p = parental generation of a cross f1 = (first filial) the...
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Mendelian Genetics Review!
• Genetic Terms:
• P = parental generation of a cross
• F1 = (first filial) the first generation after the parental (the results of the first cross)
• F2 = (second filial) the offspring from crossing two F1’s
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Mendel’s Law of Segregation• Alleles = alternative forms of the same gene
(P= purple, p= white)• Each individual has 2 alleles for every gene• Each parent passes one allele for each gene
to his/her offspring• What process does this law represent?• Meiosis• What stage of meiosis?• Anaphase I
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Genetic terms
• Homozygous = 2 identical alleles for a gene
• Heterozygous = 2 different alleles for a gene
• Dominant = The allele that is expressed in the heterozygous individual
• Recessive = the allele that is masked in the heterozygous individual
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Setting up the punnett square• The possible gametes of each individual
are determined.• Each gamete will have one of the two
alleles of the parent.• Generate a punnett square and write the
alleles for parent 1 on the left side of the Punnett square.
• Place the alleles for parent 2 above the square
• Fill the squares for each parent.
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Interpreting the results of a Punnett square
• Genotypes that resulted from this monohybrid cross (Ss x Ss)
•
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The test cross• How can you determine the genotype of a
dominant individual?
• In sheep, white is dominant to black
• It is customary to determine the letter for the alleles of a given gene by using the first letter of the dominant allele.
• Therefore W = white and w = black
• How can you tell if white sheep is homozygous or heterozygous?
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• Conduct a test cross, where the unknown dominant individual is actually mated with the known recessive .
• W ___ x ww
• After mating, look at the offspring’s phenotype to determine the unknown parent’s genotype.
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Dihybrid Cross
• R = red r = white
• T = tall t = short
• What is the F2 phenotypic ratio of offspring of a cross between pure red, tall plants and pure white, short plants?
• Punnett Square Approach
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Law of Independent Assortment
• What a parent gives for flower color has nothing to do with what a parent gives for plant height. Genes are assorted independently of each other.
• What assumption did Mendel make for this law to work?
• The traits occur on different chromosomes!
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Incomplete Dominance
• One allele is not completely dominant over the other, so the heterozygote has an intermediate phenotype between the 2 homozygotes
• In snapdragons, red snapdragons crossed with white will produce all pink flowers.
• Write your alleles like thisR’= redR= white
RR’= pink
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Codominance
• Both alleles are dominant and are expressed equally
• A black chicken and a white chicken will produce a checkered chicken
• Write your alleles like this – B= black– W= white– BW= checkered
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ProbabilityRule of multiplication
• The odds of 2 or more independent events occurring together is the product of each independent event.
• In a cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive?
• ½ x ½ = ¼
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Rule of addition
• Probability that events that can occur in 2 or more different ways will occur, is the sum of the separate problems.
• In a cross between pea plants that are heterozygous for flower color (Pp), what is the probability of the offspring being heterozygous?
• ¼ + ¼ = ½
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• What is the probability that a cross between 2 organisms with genotypes AaBbCc and AaBbCc will produce an offspring with aabbcc?
• 1/4 x 1/4 x 1/4 = 1/64
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Multiple Alleles
Blood Type
Genotypes Antigens in blood
Antibodies in serum
A IAIA, or
IAi
A Anti B
B IBIB, or
IBi
B Anti A
AB IAIB A, B ---
O ii --- Anti A and Anti B
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Terms
• Plietropy = multiple effects from a single gene
• Epistasis = one locus alters the phenotypic expression of a second gene
• Polygenic inheritance = additive effect of 2 or more genes on a single trait
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Pedigrees
• Family trees used to track human traits.
• Male =
• Female =
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Genetic Screening
• Carrier recognition
• Fetal testing:– Amniocentesis– CVS
• Ultrasound
• Fetoscopy