Linked • Genes on the same chromosome One homologous pair are said to be linked . They are of chromosomes inherited together as a unit and do not undergo independent Oocyte Linkage & Pedigrees assortment. • Linkage can alter expected genotype and ratios in the offspring. • In this example, only two types of Meiosis gamete are produced instead of the expected four kinds if the genes were assorted independently.

Gametes Ab Ab aB aB Genes A and B control different traits and are on the same chromosome

Linked Genes Linked Genes Parent 1 (2N) Parent 2 (2N) • The inheritance patterns involving Chromosomes • Genes located on the same AB Linked ab X after replication ab linked genes do not chromosome are said to be Linked ab follow expected linked (e.g. genes A and B). Meiosis Two genes are linked when they are Line indicates Mendelian ratios. • Linked genes tend to be on the same chromosome linkage • In this example of Gametes (N) inherited together. linked genes, only Only one gamete • Linkage results in fewer two kinds of from each replicated genetic combinations of chromosome is genotype in offspring (compared shown combinations occur to genes on separate Chromosome pair before replication in the offspring. chromosomes). • Without linkage, the • In describing linkage, the same parents would appropriate notation shows a provide four possible Possible offspring horizontal line separating genotypes: AaBb, Only two genotype linkage groups. combinations occur Aabb, aaBb, aabb.

AaBb AaBb aabb aabb Chromosomes after replication

Linked Genes in Drosophila Linked Genes in Drosophila (cont.) • The inheritance of X • The genes for these linked genes wing shape Inheritance of linked genes in Drosophila in Drosophila Wild type female Mutant male Straight wing Curled wing (straight or The genes for produces a Gray body Ebony body curled) and body wing shape and body color are greater proportion Cucu Ebeb cucu ebeb color (grey or linked (they are of parental types on the same Meiosis ebony) are on chromosome). than would be Gametes from female fly (N) Gametes from male fly (N) the same Parent Wild type Mutant male expected if the female CuEb cueb cueb cueb Straight wing Curled wing chromosome and Phenotype genes were on are inherited Gray body Ebony body Genotype Cucu Ebeb cucu ebeb separate together: chromosomes (assorting independently). CucuEbeb CucuEbeb cucuebeb cucuebeb Sex of offspring Linkage notation Cu Eb cu eb is irrelevant in cu eb cu eb this case

1 Pedigree Analysis A Pedigree • Pedigree analysis is a way of illustrating inheritance patterns. It is a good way to follow the inheritance of genetic disorders through generations. • Symbols are used to A circle represents A square represents a female. a male. represent males, Normal female Affected male females etc. For A horizontal line connecting A vertical line and a bracket connect the Sex unknown Died in infancy a male and female traits of interest, represents a marriage. parents to their children. symbols can be A half-shaded circle shaded to indicate Affected female Normal male or square indicates that a person is a individuals carrying carrier of the trait. A circle or square Non- Identical that is not shaded the trait. identical indicates that a twins • Individuals are twins A completely person neither shaded circle or expresses the trait designated by their Carrier square indicates nor is a carrier of that a person the trait. generation number (heterozygote) expresses the and then their order trait. number in that I, II, III Generations generation. 1, 2, 3 Children (in birth order)

Pedigree Chart Symbols Autosomal Recessive Traits • In this example, the arrow indicates the propositus or person through which the pedigree was discovered. • The pedigree for albinism (lack of pigment in the hair, skin and eyes) is inherited as an autosomal recessive trait. Carrier (heterozygote) • The trait is not sex linked and is shown by both males and females. The affected female in the third generation has phenotypically normal parents.

Autosomal Recessive Traits (cont.) Autosomal Dominant Traits

• All generation II offspring are carriers for the albino • In the inheritance of autosomal dominant . traits, each affected individual will have at • III-2 is an albino girl whose paternal grandmother least one parent who is also affected. and maternal grandfather are also albinos. • If a large number of families are examined • All her other relatives are phenotypically normal. about equal numbers of males and females are affected.

2 Autosomal Dominant Traits (cont.)

• Inheritance of the rare trait woolly hair in humans follows this • Sex linkage refers to the pattern. phenotypic expression of an • In the first generation, female I-2 is heterozygous for the woolly allele that is dependent on the hair allele. sex of the individual and is X • Half of her offspring are also heterozygous for the woolly hair directly tied to the sex allele, which then is expressed. The rest (II-5, II-6, II-7) are chromosomes. Y homozygous recessive and have normal hair. • Most sex linked genes are present on the (X-linkage ) and have no corresponding allele on the smaller male chromosome. • In some cases, a phenotypic trait is determined by an allele on the . Because the Y Note the size differences between the X chromosome is small and does and Y chromosomes. The Y lacks alleles not contain many genes, few for many of the genes present on the X. traits are Y-linked and Y-linked diseases are rare.

Sex Linkage X-Linked Recessive Traits Unaffected Carrier father mother • Sex-linked traits show a • Hemophilia is an X-linked disorder in which blood distinct pattern of clotting time is prolonged. inheritance. • Fathers pass sex-linked • Women who are heterozygotes are carriers for the alleles to all their recessive allele but do not have hemophilia. They can daughters but not to their XY XX sons. pass the allele to their sons (XY) who will express the • Mothers can pass sex- recessive allele and have hemophilia. linked alleles to both sons and daughters. • In females, sex-linked recessive traits will be expressed only in the homozygous condition. • In contrast, any male X Y X X X X XY receiving the recessive allele from his mother will express the trait.

Unaffected Unaffected Carrier Affected son daughter daughter son

X-Linked Recessive Traits (cont.) X-Linked Dominant Traits

• In the first generation, the female of the affected family is a • In this rare pattern of carrier for the hemophilia allele. • Two of the offspring of the affected family also carry the allele; inheritance, all the the male is affected and the female is a carrier. daughters of affected • Offspring of the female carrier and an unaffected male can be unaffected, carrier females, or affected males. males will be affected and more females than males Famously, Queen Victoria was a carrier of the allele will show the trait. for hemophilia, passing it to one of • An affected male her sons and, through her must always have an daughters, to the royal families of affected mother. Prussia, Russia, and Spain.

3 X-Linked Dominant Traits (cont.) X Inactivation

• In the nucleus of any non- • The inheritance of a rare form of follows this dividing somatic cell, one of the B = dominant allele (black fur) inheritance pattern. X chromosomes condenses to Y = recessive allele (yellow fur) form a visible piece of chromatin, inactive alleles • The male I-2 is affected and all his daughters II-2, II-3, XB XY = called a Barr body . on Barr bodies and II-4 are affected. • The chromosome is inactivated, so that only one X chromosome • The affected female II-4 can produce affected in a cell ever has its genes offspring of both sexes (III-2, III-3). expressed. • The inactivation is random. In XB XY XB XY any cell, the inactive X may be either the maternal homologue or the paternal homologue. • Random X inactivation is responsible for the tortiseshell and calico coloring in domestic cats. • The presence of Barr bodies is important in viable human aneuploidies .

Autosomal & Sex Linked Genes Autosomal & Sex Linked Genes

• Genes on one or other of the sex chromosomes Autosomal Genes Sex-Linked Genes produce inheritance patterns different from that shown by . They also show different 1. All individuals carry two 1. Males carry only one allele of results with a reciprocal cross. alleles of each each gene (hemizygous) Reciprocal Cross 2. operates in 2. Dominance operates in A reciprocal cross involves a pair of crosses in which the of the partners are reversed: both males and females females only. 3. Reciprocal crosses 3. Reciprocal crosses produce

Reciprocal produce the same results different results. XXcross 4. Alleles passed equally to 4. ‘Criss-cross’ inheritance Colorblind Normal Normal Colorblind female male female male male and female offspring pattern: father to daughter to grandson, etc Different results from the two crosses indicate that sex-linkage of the gene is controlling the trait Reciprocal XXcross Colorblind Normal Normal Colorblind female male female male

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