Inheritance

Gregor Mendel

An Austrian monk, who in 1860 developed certain laws of heredity (rules for passing of traits from one generation to the next)

Gregor Mendel investigated genetics at the organismal level. He experimented with pea plants and their offspring.

Mendel's goal was to understand how traits are passed from one generation to the next.

What is a trait?

By quantifying the patterns in which traits (your inherited characteristics) are inherited, Mendel developed two "laws".

The law of segregation - states that each individual has two factors for each trait but can only pass on one to

their kids. - today we call the traits genes and the

factors alleles. (alternative forms of a gene)- alleles have the same position on a pair of homologous chromosomes (chromosomes for the same traits)

Mendel’s second law: The law of independent assortmentThe alleles segregate (separate)during the formation of the gametes (egg and sperm)and each gamete has only one allele from each pair the parent possesses . Each chromosome is passed on independent of the others (you have 23 pairs, 22 autosome and 1 sex chromosome)

Fertilization gives each new individual two alleles for each trait again.

Alleles are alternative forms of a gene located at one site on a chromosome; alleles determine the traits of individuals (your genotype and your phenotype).

Chromosomes and their alleles separate and assort independently when gametes form; this increases variety among offspring.

Summary

Gene loci - located on homologous chromosomes

How do we inherit a single trait?

Dominant allele - it is expressed when present- it is designated with a capital (uppercase) letter - an example is W for widow’s peak.

Recessive allele- it is only expressed in the absence of a dominant

allele.- it is designated with a lowercase letter indicates - An example is w for continuous hairline.

We want to know the mode of inheritance.

Widow’s peak

WW or Ww

ww

Genotype versus Phenotype

Genotype refers to the genes of an individual which can be represented by two letters or by a short descriptive phrase.

Homozygous means that both alleles are the same; for example,

WW stands for homozygous dominant ww stands for homozygous recessive.

Heterozygous means that the members of the allelic pair are different

for example, Ww, a heterozygote

Phenotype refers to the physical or observable characteristics of the individual.

Both WW and Ww result in widow’s peak, - the phenotype is a widow's peak- however, we have two genotypes resulting in

the same phenotype.

ww results in a straight hairline- in this case, the phenotype can only result from

one genotype

Gamete Formation

Because homologous pairs separate during meiosis, a gamete has only one allele from each pair of alleles.

If the allelic pair is Ww, a gamete would contain either a W or a w, but not both.

Ww represents the genotype of an individual.

Gametes are represented by W or w.

One-Trait Crosses

In one-trait crosses, only one trait such as type of hairline is being considered.

When performing crosses, the original parents are called the parental generation, or the P generation.

All of their children are the filial generation, or F generation.

In a one-trait cross, if one parent is homozygous dominant and the other is homozygous recessive, all offspring of the filial generation with be heterozygous for that trait.

If you know the genotype of the parents, it is possible to determine the traits of the gametes.

A Punnett square can then be used to determine the phenotypic ratio among the offspring.

When a heterozygote (monohybrid) reproduces with another heterozygote (monohybrid), the phenotypic ratio of the offspring will be 3 expressing the dominant trait to 1 expressing the recessive trait (3:1 ratio).

A 3 : 1 ratio means that there is a 75% chance of the dominant phenotype and a 25% chance of the recessive phenotype.

Monohybrid cross

One-Trait Crosses and Probability

Laws of probability alone can be used to determine results of a cross.

The laws are:

1) the probability that two or more independent events will occur together is the product of their chances occurring separately.

2) the chance that an event that can occur in two or more independent ways is the sum of the individual chances.

In the cross of Ww x Ww, what is the chance of obtaining either a W or a w from a parent?

Chance of W = ½, or chance of w = ½

The probability of these genotypes is:

The chance of WW = ½ x ½ = ¼The chance of Ww = ½ x ½ = ¼The chance of wW = ½ x ½ = ¼The chance of ww = ½ x ½ = ¼

The chance of widow’s peak (WW, Ww, wW) is ¼ + ¼ + ¼ = ¾ or 75%.

The One-Trait Testcross

PROBLEM: It is not always possible to discern a homozygous dominant from a heterozygous

individual by inspection of phenotype.

A testcross crosses the dominant phenotype with the recessive phenotype.

If a homozygous recessive phenotype is among the offspring, the parent must be heterozygous.

One-trait testcross

Possibility I

Possibility II

Independent AssortmentThe law of independent assortment states that each pair of alleles segregates independently of the other pairs and all possible combinations of alleles can occur in the gametes.

This law is dependent on the random arrangement of homologous pairs at metaphase.

The Inheritance of Many Traits

Segregation and independent assortment

In two-trait crosses, genotypes of the parents require four letters because there is an allelic pair for each trait.

Gametes will contain one letter of each kind in every possible combination.

Crossing individuals who are heterozygous for two traits can produces four phenotypes.

The ratio of these four phenotypes will be in a 9:3:3:1.

Two-Trait Crosses

Two-Trait Crosses and Probability

It is possible to use the two laws of probability to arrive at a phenotypic ratio for a two-trait cross without using a Punnett square.

The results for two separate monohybrid crosses are as follows:

Probability of widow’s peak = ¾Probability of short fingers = ¾Probability of straight hairline = ¼Probability of long fingers = ¼

The probabilities for the dihybrid cross:

Probability of widow’s peak and short fingers= ¾ x ¾ = 9/16

Probability of widow’s peak and long fingers = ¾ x ¼ = 3/16

Probability of straight hairline and short fingers= ¼ x ¾ = 3/16

Probability of straight hairline and long fingers= ¼ x ¼ = 1/16

Genetic Disorders

Patterns of Inheritance

When studying human disorders, biologists often construct pedigree charts to show the pattern of inheritance of a characteristic within a family.

The particular pattern indicates the manner in which a characteristic is inherited.

Genetic counselors construct pedigree charts to determine the mode of inheritance of a condition.

Autosomal Recessive Disorders

Tay-Sachs Disease

Tay-Sachs disease is common among United States Jews of central and eastern European descent. An affected infant develops neurological impairments and dies by the age of three or four.

Tay-Sachs results from a lack of hexosaminidase A and the storage of its substrate in lysosomes.

Cystic Fibrosis

Cystic fibrosis is the most common lethal genetic disorder among Caucasians.

A chloride ion transport protein is defective in affected individuals.

Normally when chloride ion passes through a membrane, water follows.

In cystic fibrosis patients, a reduction in water results in a thick mucus which accumulates in bronchial passageways and pancreatic ducts.

Phenylketonuria (PKU)

Individuals with phenylketonuria lack an enzyme needed for the normal metabolism of phenylalanine, coded by an allele on chromosome 12.

Newborns are regularly tested for elevated phenylalanine in the urine.

If the infant is not put on a phenylalanine-restrictive diet in infancy until age seven when the brain is fully developed, brain damage and severe mental retardation result.

Autosomal Dominant Disorders

Neurofibromatosis

Small benign tumors, made up largely of nerve cells, occur under skin or on various organs.

The effects can range from mild to severe, and some neurological impairment is possible; this disorder is variably expressive.

The gene for this trait is on chromosome 17.

Huntington Disease

Individuals with Huntington disease experience progressive degeneration of the nervous system and no treatment is presently known.

Most patients appear normal until middle age.

The gene coding for the protein huntingtin contains many more repeats of glutamines than normal.

Polygenic and Multifactorial Inheritance

Polygenic traits are governed by more than one gene pair.

Several pairs of genes may be involved in determining the phenotype.

Multifactorial traits are governed by more than one gene and environmental factors.

Height is a multifactorial trait.

Skin Color

The inheritance of skin color, determined by an unknown number of gene pairs, is a classic example of multifactorial inheritance.

A range of phenotypes exist and several possible phenotypes fall between the two extremes of very dark and very light.

The distribution of these phenotypes follows a bell-shaped curve.

Multifactorial traits and disorders

Many human traits, like allergies, schizophrenia, hypertension, diabetes, cancers, and cleft lip, appear to be due to the combined action of many genes plus environmental influences.

Many behaviors, such as phobias, are also likely due to the combination of genes and the effects of the environment.

Multiple Allelic Traits

Inheritance by multiple alleles occurs when more than two alternative alleles exist for a particular gene locus.

A person’s blood type is an example of a trait determined by multiple alleles.

Each individual inherits only two alleles for these genes.

ABO Blood Types

A person has two alleles. They can be:- two A antigen (blood type A) - two B antigen (blood type B)- one each of A and B antigens (blood type AB)- or the O antigen (blood type O)

Human blood types can be type A (IAIA or IA i) type B (IBIB or IBi) type AB (IAIB) or type 0 (ii).

Incompletely Dominant Traits

Codominance means that both alleles are equally expressed in a heterozygote.

Incomplete dominance is exhibited when the heterozygote shows not the dominant trait but an intermediate phenotype, representing a blending of traits.

Such a cross would produce a phenotypic ratio of 1:2:1.

Incomplete dominance

Sickle-Cell Disease

Sickle-cell disease is an example of a human disorder controlled by incompletely dominant alleles.

Sickle cell disease involves irregular, sickle shaped red blood cells caused by abnormal hemoglobin.

HbA represents normal hemoglobinHbS represents the sickled condition.

HbA/HbA individuals are normal;

HbS/HbS individuals have sickle-cell disease

HbA/HbS individuals have the intermediate condition called sickle-cell trait.

Heterozygotes have an advantage in malaria-infested Africa because the pathogen for malaria cannot exist in their blood cells.

This evolutionary selection accounts for the prevalence of the allele among African Americans.