Mendel and Heredity Biology Ch. 8 Ms. Haut. Pre-Mendelian Theory of Heredity Blending Theory—hereditary material from each parent mixes in the offspring

Mendel and Heredity

Biology

Ch. 8

Ms. Haut

Pre-Mendelian Theory of Heredity

Blending Theory—hereditary material from each parent mixes in the offspring Individuals of a population should reach a

uniform appearance after many generations Once traits are blended, they can no longer be

separated out to appear in later generations

Pre-Mendelian Theory of Heredity

Problems—inconsistent with observations: Individuals of a population don’t reach uniform

appearance Traits can skip generations

Modern Theory of Heredity

Based on Gregor Mendel’s fundamental principles of heredity Parents pass on discrete inheritable factors

(genes) to their offspring These factors remain as separate factors from

one generation to the next

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)

Mendel’s Discoveries

Developed true-breeding lines—populations that 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

Crosses Tracking One Characteristic: Flower Color

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

First Law of Genetics: Law of Segregation alternate forms of genes are responsible for variations in

inherited traits for each trait, an organism inherits 2 alleles, one from

each parent If 2 alleles differ, one is fully expressed (dominant

allele); the other is completely masked (recessive allele) 2 alleles for each trait segregate during gamete

production

Mendel’s Principles of Heredity

Second Law of Genetics: Law of Independent Assortment During gamete formation, the segregation of the

alleles of one allelic pair is independent of the segregation of another allelic pair

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



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:


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.


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

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

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

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 extractedTest 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 placentaKaryotype-can see chromosome abnormalities

Ultrasound at 12 weeks--can see any physical abnormalities

Variations to Mendel’s First Law of Genetics

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

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

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

Polygenic Traits

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

Discovery of Sex-Linkage

Thomas Hunt Morgan provided convincing evidence for Chromosomal Theory of Inheritance

Experiments with Drosophila revealed sex-linkage traits. Why Drosophila? Easily cultured Prolific breeders Short generation times Only 4 pairs of chromosomes, visible under microscope

Chromosomal Basis of Sex Varies with Organism

Fruit flies (Drosophila), like Mammals use an X-Y system XX = female, XY = male Y chromosome has many fewer genes than X chromosome Y chromosome has SRY gene, determines testes development

Sex-Linked Genes have Unique Patterns of Inheritance

For genes on X chromosomes, females have 2 copies of gene—can have 2 different alleles

For genes on X chromosomes, males have only one allele; the allele they express Males’ X comes from mom (dad contributes Y) Males are said to be hemizygous If allele is recessive, it will be expressed

Genes on X chromosome are said to be sex-linked Males are more likely to have disorders that are

inherited as sex-linked recessives

Sex-Linked Recessive Trait

F1 Generation: All red eyes

F2 Generation: 3 red eyes: 1 white eyes But, the recessive trait—white eyes—was linked to sex.

•All females had red eyes•½ males had red eyes and ½ had white eyes

The gene responsible is located on the X chromosome

Documents

Mendel and Heredity Biology Ch. 8 Ms. Haut. Pre-Mendelian Theory of Heredity Blending Theory—hereditary material from each parent mixes in the offspring