Mendelian Genetics

AP Biology

Ch. 14

Ms. Haut

Pre-Mendelian Theory of Heredity

• Blending Theory—hereditary material from each parent mixes in the offspring

1. Individuals of a population should reach a uniform appearance after many generations

2. Once traits are blended, they can no longer be separated out to appear in later generations

• Problems—inconsistent with observations:

1. Individuals of a population don’t reach uniform appearance

2. Traits can skip generations

Modern Theory of Heredity

• Based on Gregor Mendel’s fundamental principles of heredity

1. Parents pass on discrete inheritable factors (genes) to their offspring

2. These factors remain as separate factors from one generation to the next

Mendel’s Discoveries

• Developed pure lines—populations that “breed true” (always produce offspring with the same traits as the parents when parents are self-fertilized)

• Counted his results and kept statistical notes on experimental crosses

Mendel’s Principles of Heredity

1. First Law of Genetics: Law of Segregationa) alternate forms of genes are responsible for

variations in inherited traits

b) for each trait, an organism inherits 2 alleles, one from each parent

c) If 2 alleles differ, one is fully expressed (dominant allele); the other is completely masked (recessive allele)

d) 2 alleles for each trait segregate during gamete production

Useful Genetic Vocabulary

• Homozygous—having 2 identical alleles for a given trait (PP or pp)

• Heterozygous—having 2 different alleles for a trait (Pp); ½ gametes carry one allele (P) and ½ gametes carry the other allele (p)

• Phenotype—an organism’s expressed traits (purple or white flowers)

• Genotype—an organism’s genetic makeup (PP, Pp, or pp)

•Combinations resulting from a genetic cross may be predicted by a Punnett square•This law predicts a 3:1 ratio observed in the F2 generation of a monohybrid cross

x

x

x

x

x

x

x

Ratio3.15:1

3.14:1

3.01:1

2.96:1

2.95:1

2.82:1

2.84:1

3:1

PP(homozygous)

Pp(heterozygous)

Pp(heterozygous)

pp(homozygous)

1

2

1 White

3

1

Purple

Purple

Purple

Genotypic Ratio 1:2:1 Phenotypic Ratio 3:1

Genotype versus Phenotype

The Testcross

• The cross of any individual to a homozygous recessive parent

• Used to determine if the individual is homozygous dominant or heterozygous

CAUTION:Must perform many, many crosses to be statistically significant

Mendel’s Principles of Heredity

2. Second Law of Genetics: Law of Independent Assortment

a) During gamete formation, the segregation of the alleles of one allelic pair is independent of the segregation of another allelic pair

b) Law discovered by following segregation of 2 genes

Dihybrid Cross

Mendelian Inheritance Reflects Rules of Probability• Rules of Multiplication: The probability that

independent events will occur simultaneously is the product of their individual probabilities.

Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive?

Answer: • Probability that an egg from the F1 (Pp) will

receive a p allele = ½ • Probability that a sperm from the F1 will receive a

p allele = ½ • Overall probability that 2 recessive alleles will

unite at fertilization: ½ x ½ = ¼

Mendelian Inheritance Reflects Rules of Probability

Question: For a dihybrid cross, YyRr x YyRr, what is the probability of an F2 plant having the genotype YYRR?

Answer: • Probability that an egg from a YyRr parent will

receive the Y and R alleles = ½ x ½ = ¼ • Probability that a sperm from a YyRr parent will

receive the Y and R alleles = ½ x ½ = ¼ • Overall probability of an F2 plant having the

genotype YYRR: ¼ x ¼ = 1/16

Works for Dihybrid Crosses:

Mendelian Inheritance Reflects Rules of Probability

• Rules of Addition: The probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways.

Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will being a heterozygote?

Answer: • There are 2 ways in which a heterozygote may

be produced: the dominant allele may be in the egg and the recessive allele in the sperm, or the dominant allele may be in the sperm and the recessive allele in the egg.

Mendelian Inheritance Reflects Rules of Probability

• Probability that the dominant allele will be in the egg with the recessive in the sperm is ½ x ½ = ¼

• Probability that the dominant allele will be in the sperm with the recessive in the egg is ½ x ½ = ¼

• Therefore, the overall probability that a heterozygote offspring will be produced is ¼ + ¼ = ½

Variations to Mendel’s First Law of Genetics

1. Incomplete dominance—pattern of inheritance in which one allele is not completely dominant over the other

• Heterozygote has a phenotype that is intermediate between the phenotypes of the 2 homozygous dominant parent and homozygous recessive parent

Incomplete Dominance in Snapdragon Color

Genotypic ratio:

Phenotypic ratio:

1 CRCR: 2 CRCW: 1 CWCW

1 red: 2 pink: 1 white

F2

Variations to Mendel’s First Law of Genetics

2. Codominance—pattern of inheritance in which both alleles contribute to the phenotype of the heterozygote

Codominance in MN Blood Groups

• MN blood group locus codes for the production of surface glycoproteins on the red blood cell• There are 3 blood types: M, N, and MN

Blood Type Genotype

M MM

N NN

MN MN

The MN blood type is the result of full phenotypic expression of both alleles in the heterozygote; both molecules, M and N, are produced on the red blood cell

Pedigree Analysis

• Analysis of existing populations

• Studies inheritance of genes in humans

• Useful when progeny data from several generations is limited

• Useful when studying species with a long generation time

= female

= male

= affected individual

= mating

= offspring in birth order I and II are generations

I

II

Symbols:

= Identical twins

= Fraternal twins

Dominant Pedigree:

I

II

III

For dominant traits:•Affected individuals have at least one affected parent•The phenotype generally appears every generation•2 unaffected parents only have unaffected offspring

Recessive Pedigree:

I

II

III

For recessive traits:•Unaffected parents can have affected offspring•Affected progeny are both male and female

Multiple Alleles

• Some genes may have more than just 2 alternate forms of a gene.

• Example: ABO blood groups– A and B refer to 2 genetically determined

polysaccharides (A and B antigens) which are found on the surface of red blood cells (different from MN blood groups)

• A and B are codominant; O is recessive to A and B

Multiple Alleles for the ABO Blood Groups

3 alleles: IA, IB, i

Pleiotropy

• The ability of a single gene to have multiple phenotypic effects (pleiotropic gene affects more than one phenotype)

• Example:•In tigers and Siamese cats, the gene that controls fur pigmentation also influences the connections between a cat;s eyes and the brain. A defective gene cause both abnormal pigmentation and cross-eye condition

Epistasis

• Interaction between 2 nonallelic genes in which one modifies the phenotypic expression of the other.

• If epistasis occurs between 2 nonallelic genes, the phenotypic ratio resulting from a dihybrid cross will deviate from the 9:3:3:1 Mendelian ratio

CC, Cc = Melanin depositioncc = AlbinismBB, Bb = Black coat colorbb = Brown coat color

A cross between heterozygous black mice for the 2 genes results in a 9:3:4 phenotypic ratio 9 Black (B_C_) 3 Brown (bbC_) 4 Albino (__cc)

Polygenic Traits• Mode of inheritance in which the additive effect of 2 or more genes determines a single phenotypic character

• Skin pigmentation in humans --3 genes with the dark-skin allele (A, B, C) contribute one “unit” of darkness to the phenotype. These alleles are incompletely dominant over the other alleles (a, b, c) --An AABBCC person would be very dark; an aabbcc person would be very light --An AaBbCc person would have skin of an intermediate shade

Nature versus Nurture• Environmental conditions can influence the phenotypic expression of a gene, so that a single genotype may produce a range of phenotypes

• One may have a history of heart disease in their family and thus be at risk of heart disease themselves. If this person watches his/her diet, exercises, doesn’t smoke, etc. his/her risk of actually developing heart disease decreases

Recessive Human Disorders

• Parents are generally unaffected

• Defective form of a normal trait. Generally, more serious phenotypic affect than dominant genes

• 2 Heterozygous normal, unaffected parents can have affected offspring

• Probability the child of 2 carriers will be:– affected = ¼ – Normal, but carriers = 1/2

Recessive Human Disorders

• Cystic Fibrosis; autosomal recessive– Ineffective component of Na+/Cl-; affects

glands that produce mucus

• Tay-Sachs; autosomal recessive– Usually fatal by 2 or 3 yrs– Developmental retardation, followed by

paralysis, dementia, and blindness– Lack enzyme to breakdown lipids—

accumulate in brain so cells lose function

Recessive Human Disorders

• Sickle-cell anemia; autosomal recessive– Caused by single amino acid substitution in

hemoglobin– Abnormal hemoglobin packs together to

form rods creating crescent-shaped cells– Reduces amount of oxygen hemoglobin

can carry

Dominant Human Disorders

• Traits inherited in every generation

• When there is 1 affected parent; ½ progeny are affected

• 2 affected parents can have unaffected offspring

• If prevents survival, then gene is quickly eliminated from population

• Usually more variable in its effects. If lethal, usually after reproductive age

Dominant Human Disorders

• Huntington’s Disease; autosomal dominant• Average onset is 40 yrs.• Late acting, presents itself after reproductive

age; lethal• Affects nervous system, muscle spasms• Destroys neurons• Located on chromosome 4• Children of an afflicted parent have a 50%

chance of inheriting the lethal dominant allele

Recessive Pedigree

Genetic Testing & Counseling

• Genetic counselors can help determine probability of prospective parents passing on deleterious genes– Genetic screening for various known

diseases alleles (gene markers)

Genetic Testing & Counseling

• Fetal testing

Amniocentesis

– needle inserted into uterus and amniotic fluid extracted

• Test for certain chemicals or proteins in the fluid that are diagnostic of certain diseases

• Karyotype-can see chromosome abnormalities

Genetic Testing & Counseling• Fetal testing

Chorion Villus Sampling– Suctions off a small amount of fetal tissue

from the chorionic villus of placenta• Karyotype-can see chromosome

abnormalities

Ultrasound at 12 weeks--can see any physical abnormalities