Genetics

Genetics is the study of heredity, the passing of traits from parents to their children. Physical traits such as eye color are inherited, as are biochemical and physiologic traits, including the tendency to develop certain diseases.

DNA strand

Transmitting an inheritance

Inherited traits are transmitted from parents to offspring in germ cells, or gametes. Human gametes are eggs, or ova, and sperm. A person’s inheritance is determined at fertilization, when ovum and sperm are united.

In the nucleus of each germ cell are structures called chromosomes. Each chromosome contains a strand of genetic material called deoxyribonucleic acid (DNA). DNA is a long molecule that’s made up of thousands of segments called genes. Each of the traits that a person inherits – from blood type to toe shape and a myriad of others in between – is carried by their genes.

Count on chromosomes

A human ovum contains 23 chromosomes. A sperm also contains 23 chromosomes; each similar in size and shape to a chromosome in the ovum. When ovum and sperm unite, the corresponding chromosomes pair up. The result is a fertilized cell with 46 chromosomes (23 pairs) in the nucleus.

Gametes do it differently

The ova and sperm are formed by a different cell-division process called meiosis. In meiosis, each of the 23 pairs of chromosomes in a cell are split. The cell then divides and each new cell that results (an ovum or sperm) receives one set of 23 chromosomes.

Location, location, location

>The location of a gene of a chromosome is called a locus. The locus of each gene is specific and doesn’t vary from person to person. This allows each of the thousands of genes in an ovum to join the corresponding genes from a sperm when the chromosomes pair up at fertilization.

Pass it on

>A person receives ones set of chromosomes and genes from each parent. This means there are two genes for each trait that a person inherits. One gene may be more influential than the other in developing a specific trait. The more powerful gene is said to be dominant and the less influential gene is recessive.

For example, a child may receive a gene for brown eyes from one parent and a gene for blue eyes from the other parent. The gene for brown eyes is dominant; the gene fro blue eyes is recessive. The dominant gene is most likely to be expressed. Therefore, the child is most likely to have brown eyes.

All about alleles

A variation of a gene and the trait it controls – such as brown, green, or blue eye color – is called an allele. When two different alleles are inherited, they’re said to be heterozygous. When the alleles are identical, they’re termed homozygous.

A dominant allele may be expressed when it’s carried by only one of the chromosomes in a pair. A recessive allele is incapable of expression unless identical alleles are carried by both chromosomes in a pair.

Let’s talk about sex

Of the 23 pairs of chromosomes in each living human cell, 22 are not involved in controlling a person’s sex; they’re called autosomes.

The two sex chromosomes of the 23rd pair determine a person’s sex. In a female, both chromosomes are relatively large and each is designated by the letter X; females have two X chromosomes. In a male, on sex chromosome is an d X chromosome and one is a smaller chromosome, designated by the letter Y.

Each gamete produced by a male contains either and X or a Y chromosome. When a sperm with an X chromosome fertilizes an ovum, the offspring is female. When a sperm with a Y chromosome fertilizes an ovum, the offspring is male.

An explanation of mutation

A mutation is a permanent change in genetic material. When a gene mutates, it produces a trait that’s different from its original trait. The mutant gene is transmitted during reproduction. Some mutations cause serious or deadly defects that occur in three different forms:

  • Single Gene Disorders
  • Chromosomal Disorders
  • Multifactorial Disorders

Single Gene Disorders

Single-gene disorders are inherited in clearly identifiable patterns. Two important inheritance patterns are called autosomal dominant and autosomal recessive. Because there are 22 pairs of autosomes and only 1 pair of sex chromosomes, most hereditary are caused by autosomal defects.In a third inheritance pattern, sex-linked inheritance, single-gene disorders are passed through the X chromosome.Also, keep the definitions of the terms dominant and recessive in mind. Dominant genes produce abnormal traits in offspring even if only one parent has the gene; recessive genes don’t produce abnormal traits unless both parents have the gene and pass them to their offspring.

Autosomal Dominant Inheritance

The autosomal dominant inheritance pattern has the following characteristics:

  • Male and female offspring are affected equally.
  • One of the parents is also usually affected.
  • If one parent is affected, half of the children will be affected.
  • If both parents are affected, all of their children will be affected.

Understanding Autosomal Dominant Inheritance

This diagram below shows the possible offspring of a parent with recessive normal genes (aa) and a parent with an abnormal dominant gene (Aa). Note that half the offspring will be affected.

Affected

Parent (Aa)

Normal

a

A

a

Parent (aa)

a

Aa Affected

aa Normal

Aa Affected

aa Normal

Marfans syndrome is an example of an autosomal dominant disorder.

Autosomal Recessive Inheritance

The autosomal recessive inheritance pattern has the following characteristics

  • Male and female offspring are affected equally.
  • If both parents are unaffected but heterozygous for the trait (Carriers), each of their offspring has a one in four chance of being affected.
  • If both parents are affected, all of their offspring will be affected.
  • If one parent is affected and the other is unaffected (Non-carrier), all of their offspring will be unaffected but will carry the defective gene.
  • If one parent is affected and the other is a carrier, half of their offspring will be affected.

Understanding Autosomal Recessive Inheritance

The diagram below shows the possible offspring of two unaffected parents, each with an abnormal recessive gene (a) on an autosome. One if four offspring will be affected, and two will be carriers who can pass the disorder on to their offspring.

Heterozygous Parent Aa

A

a
Heterozygous

Parent Aa

A

AA

Normal

Aa

Carrier

a Aa

Carrier

aa

Affected


Gitelmans syndrome is an example of an Autosomal Recessive disorder

In many cases, no evidence of the trait appears in past generations.If you enjoy clinical jargon, you can say the patient has a “negative family history.” Cystic fibrosis is an example of the type of inheritance disorder.

Think X-Linked

Some genetic disorders are caused by genes located on the sex chromosomes and are termed sex-linked.Since the Y chromosome is not known to carry disease-causing genes, the terms X-Linked and sex-linked are interchangable.

Because females receive two X chromosomes (one from the father and one from the mother), they can be homozygous for a disease allele, homozygous for a normal allele, or heterozygous.

Because males have only one X chromosome, a single x-linked recessive gene can cause disease in a male.In comparison, a female needs two copies of the diseased gene.Therefore, males are more commonly affected by x-linked recessive diseases than females.

Understanding X-linked dominant inheritance

This diagram shows the possible offspring of a normal parent and a parent with an x-linked dominant gene on the x chromosome (shown by a dot).When the father is affected, only his daughters have the abnormal gene.When the mother is affected, both male and female offspring may be affected.

Table 1

In this example you can see when the father is affected, all the daughters are affected.Sons are not affected.A good example of this is in my family My parents had 3 daughters and 1 son.My father is color blind which is an x-linked disorder.All of his daughters had sons who are color blind.My brother was unaffected.

Normal Mother
Affected X? X X
Father X

X?X

Affected Daughter

X?X

Affected Daughter

XX

Normal Son

XX

Normal Son

Table 2

When the mother is affected, both daughters and sons may also be affected

Affected Mother

Normal

X X? X

Father

X X?X

Affected Daughter

XX

Normal Daughter

X?X

Affected Son

XX

Normal Son

X-Linked Dominant Inheritance

Characteristic of X-Linked dominant inheritance include the following:

  • A person with the abnormal trait must have one affected parent
  • If a father has an x-linked dominant disorder, all of his daughters and none of his sons will be affected.
  • If a mother has an X-linked dominant disorder, there is a 50% chance that each of her children will be affected.
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