Fig. 15-1

The location of a particular can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene Fig. 15-4a

EXPERIMENT P  Generation

F1 All offspring Generation had red eyes Fig. 15-4b

RESULTS

F2 Generation Fig. 15-4c CONCLUSION w + w P X X Generation X  Y w +

w Sperm Eggs F w + w + 1 w + Generation w

+ w Sperm Eggs w + w + w + F2 Generation w w w w + Fig. 15-7

The transmission of sex linked recessive

XNXN  XnY XNXn  XNY XNXn  XnY

Sperm Xn Y Sperm XN Y Sperm Xn Y

Eggs XN XNXn XNY Eggs XN XNXN XNY Eggs XN XNXn XNY

XN XNXn XNY Xn XnXN XnY Xn XnXn XnY

(a) (b) (c) • Sex-linked genes follow specific patterns of inheritance • For a recessive sex-linked trait to be expressed – A female needs two copies of the – A male needs only one copy of the allele • Sex-linked recessive disorders are much more common in males than in females

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Partial Linkage

• Linkage is different from • Linked genes tend to be inherited together because they are located near each other on the same chromosome. Results from genes being closely linked on the same chromosome • Linked genes in genetic experiments deviate from the results expected from Mendel’s law of independent assortment. Dihybrid YYRR X yyrr Testcross to Detect Dihybrid Independent YrRr X yyrr Assort YR yr Yr yR yr

Sperm yr

YR YyRr Phenotypic ratio

1:1:1:1 yr

yyrr Ratio of parental:Recombinant Eggs 1:1 Yr Yyrr

yR yyRr Fig. 15-9-1 EXPERIMENT P Generation (homozygous)

Wild type Double mutant (gray body, (black body, normal wings)  vestigial wings)

b+ b+ vg+ vg+ b b vg vg

Morgan 1912 Fig. 15-9-2 EXPERIMENT P Generation (homozygous)

Wild type Double mutant (gray body, (black body, normal wings)  vestigial wings)

b+ b+ vg+ vg+ b b vg vg

F1 dihybrid (wild type) Double mutant TESTCROSS 

b+ b vg+ vg b b vg vg Fig. 15-9-3 EXPERIMENT P Generation (homozygous)

Wild type Double mutant (gray body, (black body, normal wings)  vestigial wings)

b+ b+ vg+ vg+ b b vg vg

F1 dihybrid (wild type) Double mutant TESTCROSS 

b+ b vg+ vg b b vg vg

Testcross offspring Eggs b+ vg+ b vg b+ vg b vg+

Wild type Black- Gray- Black- (gray-normal) vestigial vestigial normal b vg

Sperm

b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg Fig. 15-9-4 EXPERIMENT P Generation (homozygous)

Wild type Double mutant (gray body, (black body, normal wings)  vestigial wings)

b+ b+ vg+ vg+ b b vg vg

F1 dihybrid (wild type) Double mutant TESTCROSS 

b+ b vg+ vg b b vg vg

Testcross offspring Eggs b+ vg+ b vg b+ vg b vg+

Wild type Black- Gray- Black- (gray-normal) vestigial vestigial normal b vg

Sperm

b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg PREDICTED RATIOS

If genes are located on different chromosomes: 1 : 1 : 1 : 1

If genes are located on the same chromosome and parental are always inherited together: 1 : 1 : 0 : 0

RESULTS 965 : 944 : 206 : 185 Fig. 15-10 Testcross Gray body, normal wings Black body, vestigial wings parents (F1 dihybrid) (double mutant) b+ vg+ b vg

b vg b vg Replication Replication of chromo- of chromo- somes somes b+ vg+ b vg

b+ vg+ b vg b vg b vg

b vg b vg Meiosis I

b+ vg+ Meiosis I and II b+ vg b vg+

b vg Meiosis II

Recombinant chromosomes

b+ vg+ b vg b+ vg b vg+ Eggs

Testcross 965 944 206 185 offspring Wild type Black- Gray- Black- (gray-normal) vestigial vestigial normal b vg b+ vg+ b vg b+ vg b vg+

b vg b vg b vg b vg Sperm

Parental-type offspring Recombinant offspring

Recombination 391 recombinants =  100 = 17% frequency 2,300 total offspring 20% recombination

A B 40%

With crossing over

40% A B a b

a b

10% a B

10% A b Testing for Assortment/Linkage

1. Generate a dihybrid 2. Testcross the dihybrid 3. Compare the % of parental to recombinants A. If 50% parental:50% recombinant – Independent Assortment B. If more parental than recombinant – partial linkage C. If only parental and no recombinant – complete linkage • The discovery of linked genes and recombination due to crossing over led Alfred Strutevant to a method of constructing genetic maps • He assumed the farther apart genes are , the higher the probability that a cross over will happen between them and therefore the higher the recombination frequency.

The closer the two genes are on a chromosome the fewer recombinants Minimum = 0% recombinants

The further two genes are on a chromosome the more recombinants Maximum = 50% recombinants

Linkage therefore can be used as a measure of genetic distance on chromosome

1 Map Unit = 1 % recombination Gene Order on Chromosome

B – Vg 17 MU B – Cn 9 MU Vg – Cn 9.5 MU

Partial Linkage – two genes are so close on the same chromosome that recombination occurs less than 50% of the time. Complete Linkage – two genes on the same chromosome so close that recombination cannot separate them. Independent Assortment – two genes on different chromosomes or two genes on the same chromosome but far enough apart that recombinant occurs 50% of the time. Example Problem In Drosophila long wings is dominant to dumpy wings and round eyes is dominant to star eyes. A dihybrid fly was generated by mating a long wing round eye fly with a dumpy wing star eye fly. This dihybrid fly was testcrossed and the following progeny were generated. 222 long wing round eye 215 dumpy wing star eye 33 long wing star eye 30 dumpy wing round eye a. Are these genes completely linked or partially linked? b. What is the genetic distance between these two genes? c. How would the results have differed if the genes independently assorted? Exception to chromosomal Inheritance (Organellar Genes) • The inheritance of traits controlled by genes present in the chloroplasts or mitochondria – Depends solely on the maternal parent because the zygote’s cytoplasm comes from the egg – Maternal Inheritance

Pedigree Symbols Nuclear vs Organellar Human Pedigree Analysis Autosomal vs Sex Linked Multifactorial Traits

• Heart disease • Personality • IQ Alterations of chromosome number or structure cause some genetic disorders.

• So far we’ve seen that the can be affected by small scale changes involving individual genes • Random are the source of all new alleles, which can lead to a new phenotype. Abnormal chromosome #: Aneuploidy Human Aneuploids

• Trisomy 21 • – XO – turner syndrome – XXY – klinefelters – XYY

Abnormal chromosome numbers

• Polyploidy: Common in plant • ~70 % of flowering plants, • Banana are triploid, • Wheat 6n • Strawberries 8n Alterations in chromosome structure

• Meiosis errors and damaging agents such as radiation can cause breakage of the chromosome • four types of structural damage

Chromosome Structure

reciprocal translocation between 9 and 22 (Philadelphia Chromosome) Disorders caused by structurally altered chromosomes • Cri du chat – deletion in chromosome 5 • Chronic myogenous leukemia

Reciprocal Normal chromosome 9 translocation Translocated chromosome 9

Normal chromosome 22 Translocated chromosome 22

Fig. 15-8 X chromosomes Allele for orange fur Early embryo: Allele for black fur

Cell division and Two cell inactivation populations in adult cat: Active X Inactive X Active X Barr Body – inactive X visible in interphase nucleus Black fur Orange fur Genomic imprinting

• Def: a parental effect on gene expression • Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm. • : higher prevalence of disorder and retardation in males