Chapter 8: Genetics

Gregor Mendel

Gregor Mendel is known as the father is genetics. One of his hobbies was to study the effects of cross-breeding on different strains of pea plants. Mendel works exclusively with true-breeder pea plants. This means that the plants he used were genetically pure and consistently produced the same traits. For example, tall plants always produced tall plants; short plants always produced short plants.

Heterozygous

Heterozygous means having one each of two different alleles.

Homozygous

When an individual has two of the same allele, whether dominant or recessive, they are homozygous.

monohybrid cross

A monohybrid cross is a mating between individuals who have different alleles at one genetic locus of interest.

Pedigree

A pedigree is simply a chart that shows the presence of a particular phenotype in a given family, usually over several generations.

XX? XY?

A sex chromosome can either be male, in which case we call it a Y chromosome, or it can be female, in which case we call it an X chromosome. A person whose phenotype is male has the sex genotype XY, and a person whose phenotype is female has the sex genotype XX. The male got an X chromosome from his mother (whose genotype is XX) and a Y chromosome from his father (whose genotype is XY). The female got an X chromosome from her mother and an X chromosome from her father. So whether a child is born male or female depends on whether it gets an X or a Y chromosome from its father—because the father can donate either one.

Sex-Linked Traits

A sex-linked trait, is a trait whose allele is carried on one of the sex chromosomes. Almost all sex-linked traits have alleles that are carried on the X chromosome. The X chromosome is much larger than the Y chromosome and therefore carries more genes.

How does a test cross use a recessive organism?

A test cross uses a recessive organism to determine the genotype of an organism of unknown genotype.

One Last Point About X-linked Recessive Traits: They Can't Go From a Male to a Male

An X-linked recessive trait cannot go from father to son. That's because the allele for an X-linked trait is located on the X chromosome, and a father does not give his son an X chromosome. He gives him a Y chromosome.

Allele

An allele is one of the possible forms of a gene. Most genes have two alleles, a dominant allele and a recessive allele.

Dominant

An allele or a gene that is expressed in an organism's phenotype, masking the effect of the recessive allele or gene when present.

Autosomes

Any chromosome that is not a sex chromosome. Autosomal genes will always be comprised of two copies: one copy is inherited from the mother and the other copy is inherited from the father. The traits expressed in these autosomal genes are generally unaffected by gender, and the traits will follow Mendelian patterns of inheritance.

Sex Chromosomes

As you know, their cells (other than sperm and ova) have 23 pairs of homologous chromosomes. One of those 23 pairs is the pair that determines whether a person is male or female, and this pair of chromosomes is known as the sex chromosomes.

Dihybrid Cross

Dihybrid cross is a cross between two different lines (varieties, strains) that differ in two observed traits. In the Mendelian sense, between the alleles of both these loci there is a relationship of complete dominance - recessive.

Carriers

Females who are heterozygous for X-linked recessive traits are known as carriers. They don't express the trait, but they do carry the allele for the trait.

Hemophilia (X-linked trait) and Females

If a person carries the trait, he or she carries it on the X chromosome. It's also recessive, so we can describe hemophilia as being an X-linked recessive trait. It means that a female can only get hemophilia if she's homozygous for the X chromosome that carries the allele for hemophilia.

Recessive

In genetics, a recessive gene or allele is one in which the effect is not tangible, or is masked by the effects of the dominant gene. The recessive trait may be expressed when the recessive genes are in homozygous condition or when the dominant gene is not present.

How are things represented in pedigrees?

In pedigrees, males are represented by a square, and females are represented by a circle. A horizontal line connecting the two shapes shows the mating between two individuals, and a vertical line connects offspring resulting from that mating.

Codominance

It is possible, in the case of multiple alleles (i.e., more than two), for two alleles to exhibit codominance. This means that a heterozygote expresses a mixture of the traits of both alleles. This is the case, for example, when someone has the blood type AB.

The Law of Dominance

Mendel's law of dominance states that in a heterozygote, one trait will conceal the presence of another trait for the same characteristic. Rather than both alleles contributing to a phenotype, the dominant allele will be expressed exclusively.

PEDIGREE QUESTIONS: Is the condition dominant or recessive?

Recessive conditions skip generations. Pick any affected individual in the pedigree. Look back at that individual's parents. Do they have the condition? Look at that individual's offspring (if present). Do they have the condition? If neither the parents nor the offspring display the condition, the condition is recessive.

PEDIGREE QUESTIONS: Is the condition sex-linked, or is it autosomal (carried on one of the non-sex chromosomes)?

Sex-linked diseases, as we mentioned earlier, are more common in males than in females. So to determine if the condition is sex-linked, simply count the number of males and females who are affected. If there are significantly more males than females affected, the condition is sex-linked.

RATIO FOR DIHYBRID CROSS

Simply memorize the phenotype ratio of the pea plants. For the 16 offspring there are: • 9 tall and green • 3 tall and yellow • 3 short and green • 1 short and yellow That's 9 : 3 : 3 : 1.

Why are X-linked recessive diseases are much more common in males than in females?

So, when it comes to an X-linked recessive trait, a female will express the trait in her phenotype only if she is homozygous for it. However, a male will express the trait in his phenotype if he carries the allele on his X chromosome.

Genotype

The genotype is the part (DNA sequence) of the genetic makeup of a cell, and therefore of an organism or individual, which determines a specific characteristic (phenotype) of that cell/organism/individual.

The Law of Segregation

The Law of Segregation states that every individual organism contains two alleles for each trait, and that these alleles segregate (separate) during meiosis such that each gamete contains only one of the alleles.

Phenotype

The set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.

Three X-linked Traits

The three X-linked traits to know for the exam are hemophilia (a disorder of blood clotting), color blindness, and male pattern baldness.

Test Cross

To identify whether an organism exhibiting a dominant trait is homozygous or heterozygous for a specific allele, a scientist can perform a test cross. The organism in question is crossed with an organism that is homozygous for the recessive trait, and the offspring of the test cross are examined.

The Law of Independent Assortment

Traits can segregate and recombine independently of other traits.

Mendel's three principles of genetics:

the law of dominance, the law of segregation, and the law of independent assortment.