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Announcements• Problem set 2 due next week• Exam 1 next week- scantrons

required• Next week, we will do virtual fly

lab. You must purchase Biolabsonline to do lab- start soon andpractice

Objectives• Distinguish between autosomal dominant,

autosomal recessive, and sex linked traits on apedigree.

• Be able to infer possible genotypes on apedigree.

• Know about two different processes, which occurduring meiosis, which cause phenotypic ratios toconform to Mendel’s Rule of IndependentAssortment.

• Calculate map distance from a 2 point test cross.• Establish gene order from several 2 point

crosses.

Chromosomal sex determination

• Males heterogametic (XY)– Fruit flies– Humans

• Males homogametic (ZZ), femalesheterogametic (ZW)– Birds– Moths

Figure 15.8 Some chromosomal systems of sex determination

Introduction to pedigree analysis• Why conduct a pedigree analysis?

– Some organisms cannot be crossedgenetically• Long generation times• Ethics

– We wish to study a particular family line(e.g. horses or dogs)

• Pedigree analysis applies Mendeliananalysis to ‘non-genetic’ organisms.

Figure 14.16 Large families provide excellent case studies of human genetics

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How pedigrees look• boxes male, circles female• rows belong to same

generation• Mating denoted by line

between two individuals• children attached to it by

vertical line• Affected individuals are

shaded

Figure 14.14 Autosomal recessive

• trait can skip generations• if both parents affected, all children will also be affected

Figure 14.14 Autosomal dominant

• If one parent affected– at least 1/2 children affected– or all children affected

• If both parents affected some offspring may not beaffected

• trait does not skip generations

Hemophilia: An Example• Mostly males are affected• Expression skips generations- recessive

Hemophilia:Consistent with sex linkage

Males with ‘H’ allele express trait, femalescan be carriers of trait

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Sex linked traits in pedigrees• Males show trait more often than females• Fathers transmit allele to all daughters but no sons• Female carriers pass it to 50% of their sons and

daughters.– Sons with allele will show trait– Daughters will be carriers (unless the father had the

trait).

Introduction to Linkage• Linked genes are close together on the

same chromosome.• Under independent assortment, parental

gamete frequencies (P) equal recombinantfrequencies (R)

• P > R for linked genes

50% P 50% R

Independent assortment• Genes on different chromosomes or far

apart on a chromosome.• Recombinants created by independent

orientation of homologues or crossingover.

Figure 15.1 The chromosomal basis of Mendel’s laws

When genes are linked• Most F2 progeny are parental• Parental alleles will remain together

during meiosis more often when genesare closely linked.

• Recombination indicates that crossingover occurred.

• Crossing over can be measured bydetermining the proportion ofrecombinants.

Figure 15.7 A partial genetic map of a Drosophila chromosome

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Figure 15.5a Recombination due to crossing over Unnumbered Figure (page 272) Drosophila testcross

How do we determinewhether parentals andrecombinants occur in

equal frequency?

Figure 15.4 offspring number under independent assortment

Two point cross• Count proportion of recombinants in a

test cross with two loci• Assume that crossing over occurs

randomly across chromosome• Express distance in centimorgans or

map units (mu)

Map distance formula

map distance = 100 *# recombinants

total # F2 offspring

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Figure 15.5b Recombination due to crossing over Figure 15.4 testcross