mendel and heredity biology ch. 8 ms. haut. pre-mendelian theory of heredity blending...
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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
Mendelian Inheritance Reflects Rules of Probability
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.
Mendelian Inheritance Reflects Rules of Probability
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
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
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