Extensions of the Laws of Inheritance Chapter 8 Part B

Principles of Inheritance Mendel studied traits that followed a simple pattern of dominant and recessive alleles for a single characteristic Other modes of inheritance have also been described These extensions of the laws of inheritance still follow the fundamental principles of Mendelian genetics Genotypes are expressed differently for some of these crosses The gene is named For example: letter “A” could be used as the gene name The gene name can still be used to indicate dominance depending on the type of inheritance The alleles are added to the gene name as superscripts ARAR, ARAr, ArAr

Incomplete Dominance One allele isn’t fully dominant over the other allele, so the heterozygote’s phenotype is somewhere between the two homozygotes Snapdragon flower color Red flowers = RR White flowers = rr Heterozygotes = Rr These are pink The pigment produced by the red allele is diluted and thus appears pink Mendel showed that traits don’t “blend” This still holds true

Codominance Both alleles are simultaneously expressed in heterozygotes Feather color gene A Black allele AB White allele AW Genotype of the heterozygote: ABAW

Codominance Difference between typical dominant/recessive, codominance, and incomplete dominance

Multiple Alleles There can be more than two alleles for a given gene Blood type in humans has three alleles IA codominant IB codominant i recessive

Multiple Alleles There can be more than two alleles for a given gene Genotypes and Phenotypes IAIA and IAi Type A blood IBIB and IBi Type B blood IAIB Type AB blood (codominantly expressed) ii Type O blood

Epistasis Some traits are the results of interactions of two or more gene pairs At least eight genes contribute to eye color in humans Not quite a simple as Mendel’s pea plant characteristics Leads to continuous variation in traits

Epistasis Some traits are the results of interactions of two or more gene pairs Labrador coat color Gene encoding pigment black is dominant to brown Gene encoding deposition of pigment Dominant allele promotes deposition of pigment Recessive allele reduces deposition The two genes work together to determine how much of what color pigment ends up in the coat

Linked Genes Mendel showed that genes on different chromosomes sort independently Genes that are on the same chromosome are not inherited independently of each other They are linked

Linked Genes Linked genes can sort independently at a low frequency due to crossing over during meiosis

X-linked Traits The X and Y chromosomes carry different genes Recessive alleles on the X chromosome create a unique pattern of inheritance

X-linked Traits More males affected Each male receives one X and one Y chromosome Two possible genotypes: XAY, XaY Females receive two X chromosomes Three possible genotypes: XAXA, XAXa , XaXa Thus heterozygous males are affected while heterozygous females are not

X-linked Traits Affected fathers cannot pass X-linked recessive alleles to a son All children who inherit their father’s X chromosome are female

X-linked Traits Affected fathers cannot pass X-linked recessive alleles to a son Heterozygous females are the bridge between an affected male and his affected grandson

X-linked Traits Examples Red-Green color blindness Hemophilia A Duchenne Muscular Dystrophy

X-linked Traits X-linked dominant alleles that cause disorders are rarer because they tend to be lethal in male embryos

Human Inheritance Patterns Inheritance patterns in humans are typically studied by tracking observable traits that crop up in families over many generations Data is organized in pedigree charts Chart symbols

Human Inheritance Patterns Some easily observed human traits follow Mendelian inheritance patterns Controlled by a single gene Two alleles, one dominant and one recessive

Human Inheritance Patterns Tongue rolling Dominant condition (TT, Tt) Ability to roll one’s tongue Recessive condition (tt) Inability to roll the tongue

Human Inheritance Patterns Widow’s peak Dominant condition (WW, Ww) A distinct downward point in the frontal hairline Recessive condition (ww) A continuous hairline

Human Inheritance Patterns Earlobe attachment: Dominant condition (EE, Ee) Detached or free earlobes Recessive condition (ee) Earlobes attached directly to the head

Human Inheritance Patterns Autosomal Inheritance Patterns Dominant Patterns Dominant alleles are expressed in both homozygotes and heterozygotes The trait specified tends to appear in every generation

Human Inheritance Patterns Dominant Patterns Examples Achondroplasia The allele interferes with formation of the embryonic skeleton Adults average about 4’4” and have short limbs Huntington’s disease Nervous system slowly deteriorates Involuntary muscle movements increase Hutchison-Guilford Progeria Drastic premature aging

Human Inheritance Patterns Recessive Patterns Expressed only in homozygous people Traits may skip generations Heterozygotes are carriers of the allele, but do not express the trait

Human Inheritance Patterns Recessive Patterns Examples Albinism Lack of melanin Tay-Sachs disease Malfunction of a lysosomal enzyme that breaks down gangliosides Lipids accumulate to toxic levels in nerve cells Cystic Fibrosis Difficulty breathing Chronic lung infections

Summary Extensions of the Laws of Inheritance Epistasis Linked Genes Incomplete dominance Codominance Multiple Alleles Epistasis Linked Genes X-linked traits Human inheritance patterns