Chapter 9 Patterns of Inheritance. Heredity : The transmission of traits from one generation to another. Variation : Offspring are different from their parents and siblings. Genetics : The scientific study of heredity and hereditary variation.
Patterns of Inheritance
Variation: Offspring are different from their parents and siblings.
Genetics: The scientific study of heredity and hereditary variation.
Involves study of cells, individuals, their offspring, and populations.
Red flower x White flower Pink flower
Tall height x Short height Medium height
Blue bird x Yellow bird Green birds
Fair skin x dark skin Medium skin color
If blending always occurred, eventually all extremecharacteristics would disappear from the population.
Began as a science in 1860s.
Flower color Purple or white
Seed color Yellow or green
Seed shape Round or wrinkled
Pod color Green or Yellow
Pod shape Smooth or constricted
Flower position Axial or terminal
Plant height Tall or short
Varieties of Peas
Question: What will we obtain when we cross a pea plant with purple flowers with one with white flowers?
1. If blending hypothesis is true, then plants would be an intermediate color, e.g.: light purple.
2. Someplants will be purple, others will be white.
3.All plants will be purple or all plants will be white.
All Plants in the First Generation Had Purple Flowers
Purple is Dominant Over White Flower Color
All plants displayed one trait only.
Trait Varieties Offspring
Flower color Purple or white 100% Purple
Seed color Yellow or green 100% Yellow
Seed shape Round or wrinkled 100% Round
Pod color Green or Yellow 100% Green
Pod shape Smooth or constricted 100% Smooth
Flower position Axial or terminal 100% Axial
Plant height Tall or short 100% Tall
The trait that prevailed was dominant, the other recessive.
1. Results indicate that blending hypothesis is not true.
2. Only one of the two traits appeared in the first generation. He called this the dominant trait. He called the trait that disappeared the recessive trait.
1. Results indicate that the recessive trait is intact.
2. The crossbred plants with purple flowers must be carrying the genetic information to produce white flowers.
3. The crossbred plants with purple flowers are genetically different from the purebred plants, even though they look the same.
4. Must distinguish between:
Phenotype:Physical appearance of individual.
Example:Two phenotypes for flower color.
Genotype:Genetic makeup of an individual.
Not all purple flowers are genetically identical.
5. Each individual carries two genes for a given genetic trait. One gene comes from the individual’s mother, the other from the father.
There are two alternative forms of genes or hereditary units.
The alternative forms of these hereditary units are called alleles.
P: Allele for purple flowers
p: Allele for white flowers
6. In a given individual, the two genes for a given trait may be the same allele (form of a gene) or different.
Purple PP (Homozygous dominant)
Purple Pp (Heterozygous dominant)
White pp (Homozygous recessive)
7. How can we explain the consistent 3:1 phenotypic ratio in the F2 generation?
During gamete formation, the two alleles for a given trait separate (Principle of segregation). Egg or sperm cells only contain one allele for a given trait.
When a sperm and egg come together during fertilization, each one contributes one allele to the offspring, which restores the pair of alleles.
Contributes One Allele to Offspring
Used to determine the outcome of a cross between two individuals.
Heterozygotes make 1/2 P and 1/2 p gametes.
P PP Pp
Genotype: 1/4 PP, 1/2 Pp, and 1/4 pp
Phenotype: 3/4 Purple and 1/4 white
Question: What will we obtain in F2 generation, when we cross a pea plant with round yellow peas (RRYY) with one with wrinkledgreen peas (rryy)?
F1 Generation will all be round yellow (RrYy).
Possible outcomes of F2 Generation:
1. If the two traits are inherited as a package (RY and ry), then will only get yellow round and greenwrinkled peas.
2.If two traits are inherited independently, will get:
Tracking Two Characteristics
1.Principle of Independent Assortment: Genetic traits are inherited independently of one another. One trait does not affect the inheritance of the other.
2. Heterozygous individuals with yellow round peas (RrYy) from the F1 generation, will produce four types of gametes:
1/4 RY 1/4 rY 1/4 Ry 1/4 ry
instead of only two:
1/2 RY 1/2 ry
3. The offspring of a dihybrid cross displays a 9:3:3:1 phenotypic ratio:
9/16 Yellow Round (Y-R-)
3/16 Green Round (yyR-)
3/16 YellowWrinkled (Y-rr)
1/16 GreenWrinkled (yyrr)
1. There are alternative forms of genes, the units that determine heritable traits.
These alternative forms are called alleles.
Pea plants have one allele for purple flower color, and another for white color.
2. For each inherited characteristic, an individual has two genes: one from each parent.
In a given individual, the genes may be the same allele (homozygous) or they may be different alleles (heterozygous).
3. When two genes of a pair are different alleles, only one is fully expressed (dominant allele). The other allele has no noticeable effect on the organism’s appearance (recessive allele).
Purple allele for flower color is dominant
White allele for flower color is recessive
4. A sperm or egg cell (gamete) only contains one allele or gene for each inherited trait.
Principle of Segregation: Alleles segregate (separate) during gamete formation.
(When? During meiosis I)
During fertilization, sperm and egg each contribute one allele to the new organism, restoring the allele pair.
5. Principle of Independent Assortment: Two different genetic characteristics are inherited independently of each other.*
*As long as they are on different chromosomes.
Mendel did not know about meiosis, but meiosis explains this observation.
How are chromosomes shuffled during meiosis I?
Inheritance of human traits.
Most genetic diseases are recessive.
Dominant Traits Recessive Traits
Widow’s peak Straight hairline
Freckles No freckles
Free earlobe Attached earlobe
Normal Cystic fibrosis
Normal Tay-Sachs disease
Normal hearing Inherited deafness
Huntington’s Disease Normal
Dwarfism Normal height
A. Incomplete Dominance:
For some characteristics, the F1 hybrids of a true-breed cross have an intermediate phenotype between that of parents.
Incomplete dominance does not support blending, because the parental alleles are not lost.
Snapdragon flower color
Hypercholesteremia in humans
Have Intermediate Phenotypes
B. Multiple Alleles and Codominance:
For some characteristics, there are more than 2 alleles.
Example: ABO blood type.
There are three alleles that control blood type in humans.
IA: Red blood cells have carbohydrate A.
IB: Red blood cells have carbohydrate B.
i: No carbohydrate on red blood cells.
Codominance: When both alleles are present, they are both fully expressed.
IA and IB are codominant and dominant over i.
IA = IB > i
Genotype Blood Type (Phenotype)
IA IB AB (Universal acceptor)
IA IA A
IB IB B
IB I B
ii O (Universal donor)
Blood type O: Universal donor. Blood type AB: Universal acceptor
One gene affects more than 1 characteristic.
Sickle cell anemia. There are two alleles that determine hemoglobin sequence.
A: Normal hemoglobin
a: Sickle cell hemoglobin
Alleles display incomplete dominance:
Aa Sickle cell trait (Healthy. Malaria resistance)
aa Sickle cell anemia
Individuals with sickle cell anemia (Genotype: aa) have abnormal hemoglobin, which causes many different health problems:
D. Polygenic Inheritance:
Some genetic characteristics are controlled by two or more genes:
The alleles usually have an additive effect, resulting in multiple phenotypes.
Phenotypes for skin color can range from very dark to very light.
is Determined by Several Genes
Some genetic characteristics are controlled by two genes that are on the same chromosome.
These traits tend to be inherited together or display linkage.
Linked genesdo notfollow Mendel’s principle of independent assortment.
Crossing over produces new combinations of alleles on chromosomes.
F. Sex-linked Inheritance:
Some genetic characteristics are controlled by genes that are on the sex chromosomes.
These genes are inherited differently than genes on autosomes.
Females (XX) Males (XY)
The X chromosome is much larger than the Y chromosome, and contains many more genes.
The Y chromosome is very small and contains very few genes.
X-Y System in mammals:
F. Sex-linked Inheritance:
X Chromosome Genes:
Y chromosome Genes:
F. Sex-linked Inheritance:
Women can be homozygous or heterozygous for sex-linked traits.
Men only have one X chromosome, so they are hemizygous for sex-linked traits.
For this reason, males are more susceptible to X-linked diseases.
Hemophilia is a recessive X-linked disorder, in which affected individuals’ blood does not clot normally. Males and females inherit the trait differently.
Male Genotype Male Phenotype
XHY (Hemizygous) Normal
XhY (Hemizygous) Hemophiliac
Female Genotype Female Phenotype
XHXh(Heterozygous) Normal carrier
What kind of children will be born from the marriage of a normal man (XHY) and a normal woman who is a carrier of the hemophilia gene (XHXh)?
XH XH XH XHY
Xh XH Xh XhY
Daughters: All normal. 50% carriers and 50% homozygous.
Sons: 50% normal, 50% hemophiliacs.