NOTES: Ch 14, part 2 – Extending Mendelian

Genetics

The Spectrum of Dominance

● Complete dominance occurs when the phenotypes of the heterozygote and dominant homozygote are identical

● In codominance 2 dominant alleles affect the phenotype in separate, distinguishable ways

● Example: the human blood group MN

CODOMINANCE:

CODOMINANCE: ● inheritance

characterized by full expression of both alleles in the heterozygote

Example: in chickens, BB = black feathers, bb = white, Bb = “speckled” (both black and white feathers)

INCOMPLETE DOMINANCE: ● inheritance where one

allele is not completely dominant over the other, so the heterozygote has a phenotype that is intermediate between the phenotype of the 2 homozygotesEx: RR = red flowers, rr = white, Rr = pink

The Relationship Between Dominance and Phenotype

● Dominant and recessive alleles do not really “interact”

● they lead to synthesis of different proteins that produce a phenotype

Frequency of Dominant Alleles

● Dominant alleles are not necessarily more common in populations than recessive alleles

Multiple Alleles

● Most genes exist in populations in more than two allelic forms

● The ABO blood group in humans is determined by multiple alleles

Table 14.2

Polygenic Inheritance

● Many human characters vary in the population along a continuum and are called quantitative characters

POLYGENIC INHERITANCE:

● mode of inheritance in which the additive effect of 2 or more genes determines a single phenotypic characterExamples: height

hair color (4 gene pairs)eye color (2 gene pairs)skin color

AaBbCc AaBbCc

aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC

20⁄64

15⁄64

6⁄64

1⁄64

Fra

ctio

n of

pro

geny

● Quantitative variation usually indicates polygenic inheritance

(An additive effect of 2 or more genes on a single phenotype)

PLEIOTROPY: ● the ability of a single gene to have

multiple phenotypic effects

Examples: sickle cell anemia

Siamese cats & tigers

EPISTASIS: ● interaction between 2 nonallelic genes in

which one modifies the phenotypic expression of the other

Example: gene for pigment deposition is epistatic to gene for melanin production in mice

● An example of epistasis:

BC bC Bc bc1⁄41⁄41⁄41⁄4

BC

bC

Bc

bc

1⁄4

1⁄4

1⁄4

1⁄4

BBCc BbCc BBcc Bbcc

Bbcc bbccbbCcBbCc

BbCC bbCC BbCc bbCc

BBCC BbCC BBCc BbCc

9⁄163⁄16

4⁄16

BbCc BbCc

Sperm

Eggs

Nature and Nurture: The Environmental Impact on Phenotype

● Another departure from simple Mendelian genetics arises when the phenotype for a character depends on environment as well as on genotype

● The norm of reaction is the phenotypic range of a particular genotype that is influenced by the environment

Figure 14.13

MULTIFACTORIAL INHERITANCE:

● A trait depends on many factors; a variety of genotypes as well as environmental influences (such as certain chemicals, medicines, or diet)

Examples of disorders that may be a result of multifactorial inheritance: diabetes, heart disease, neural tube defects, autism, Alzheimer disease, ALS, and many cancer syndromes

~Same genotype

~Phenotype depends on acidity of soil

~height controlled by several genes

~height influenced by amount of nourishment

Integrating a Mendelian View of Heredity and Variation

● An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior

● the phenotype reflects its overall genotype and unique environmental history

Inheritance Patterns for Genetic Diseases in Humans

Pedigree Analysis● A pedigree is a family tree that describes

the interrelationships of parents and children across generations

George Arlene

Sandra Tom Sam Wilma Ann Michael

Daniel Alan Tina

Christopher

Carla

● Inheritance patterns of particular traits can be traced and described using pedigrees

Figure 14.14 A, B

Ww ww ww Ww

wwWwWwwwwwWw

WWor

Ww

ww

First generation(grandparents)

Second generation(parents plus aunts

and uncles)

Thirdgeneration

(two sisters)

Ff Ff ff Ff

ffFfFfffFfFF or Ff

ff FForFf

Widow’s peak No Widow’s peak Attached earlobe Free earlobe

(a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)

1) Autosomal Recessive:

● recessive alleles that cause human disorders are usually defective versions of normal alleles

● defective alleles code for either a malfunctioning protein or no protein at all

● heterozygotes can be phenotypically normal, if 1 copy of the normal allele is all that is needed to produce sufficient quantities of the “good” protein

Examples: cystic fibrosis, Tay-Sachs, sickle cell anemia

● “Carriers” are heterozygous individuals who carry the recessive allele but are phenotypically normal

Cystic Fibrosis

● Symptoms of cystic fibrosis include:

-Mucus buildup in the some internal organs

-Abnormal absorption of nutrients in the small intestine

Sickle-Cell Disease

● Sickle-cell disease:-Affects one out of 400 African-Americans-Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells

● Symptoms include:-Physical weakness, pain, organ damage, and even paralysis

2) Autosomal Dominant: ● only 1 dominant allele is needed in order to

produce the effects of these diseases (heterozygous)

● Lethal homozygous dominant condition results in spontaneous abortion of fetus

● homozygous recessives are of normal phenotype

Examples: achondroplasia, Huntington’s Disease

● ACHONDROPLASIA: a form of dwarfism that is lethal when homozygous for the dominant allele

Figure 14.15

● HUNTINGTON’S DISEASE: a degenerative disease of the nervous system

-it has no obvious phenotypic effects until about 35 to 40 years of age

Figure 14.16

Genetic Testing and Counseling

● Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease

Counseling Based on Mendelian Genetics and Probability Rules

● Using family histories genetic counselors help couples determine the odds that their children will have genetic disorders

Tests for Identifying Carriers

● For a growing number of diseases tests are available that identify carriers and help define the odds more accurately

Fetal Testing

● AMNIOCENTESIS: the liquid that bathes the fetus is removed and tested

● CHORIONIC VILLUS SAMPLING (CVS): a sample of the placenta is removed and tested