Heredity: Gender-Related Factors

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Chapter: Anatomy and Physiology for Health Professionals: Heredity

Gender-Related Factors - Sex Determination: Genes on Sex Chromosomes, Gender Effects on Phenotype; Chromosome Disorders: Polyploidy, Aneuploidy


Gender-Related Factors

The human somatic or nonsex cells are the X and Y chromosomes of males and the two X chromosomes of females. Each egg carries one X chromosome, while sperm carry either an X or a Y chromosome. At con-ception, sex is determined. When a Y-­bearing sperm fertilizes an egg, a male is conceived, and when an X-bearing sperm fertilizes an egg, a female is con-ceived. A male is termed the heterogametic sex since the two chromosomes are different and a female is termed the homogametic sex since the two chromo-somes are the same.


Sex Determination

Development of male characteristics is derived from a Y chromosome gene called SRY for the sex-­ determining region of the Y chromosome. The SRY gene encodes a protein called transcription factor, which then activates other genes. SRY activates gene transcription, directing development of male struc-tures in an embryo. It also suppresses formation of female structures. Absence of the SRY transcription factor along with expression of the gene called Wnt4 causes development of a female body to occur.

Genes on Sex Chromosomes

Genes that are part of X and Y chromosomes are inherited differently than autosomal genes. This is due to the different sex chromosome makeups of males and females. X-linked traits are transmitted on the X chromosome, which has more than 1,500 genes, and Y-linked traits are transmitted on the Y chromosome, which has only 231 protein-encoding genes.

There are three groups of Y-linked genes, based on similarities to X-linked genes:

One group is made up of genes at the tips of the Y chromosome that have counterparts on the X chromosome. Many proteins that function in both sexes are encoded by these genes, related to bone growth, energy metabolism, and synthesis of receptors and hormones.

Components of the second functional group of Y chromosome genes are similar, in their DNA sequence, to certain X chromosome genes. However, they are not exactly the same. The genes are expressed in almost all tissues, including those that only exist in males.

The third gene group includes those unique to the Y chromosome. Many of these control male fertility, including the SRY gene. Certain types of male infertility may develop due to deletions of these parts of the Y chromosome. In this group, other genes encode proteins participating in cell-cycle control. There are also proteins that regulate gene expression, various enzymes, and protein receptors for biochemicals linked to immunity.

Y-linked genes are only transmitted from fathers to their sons, since only males have Y chromosomes. There are differences in inheritance patterns of X-linked genes between females and males. This is linked to the fact that any gene on a male’s X chromo-some is expressed in his phenotype, since there is no second allele on a second X chromosome that masks its expression. Males are hemizygous for X-linked traits since there is only one copy of each X chromo-some gene. An example of recessive X-linked traits is red-green color blindness.

Each male inherits the Y chromosome from his father and the X chromosome from his mother. Each female inherits one X chromosome from each parent. When a mother is heterozygous for a certain X-linked gene, her son has a 50% chance of inheriting either allele from her. Therefore, X-linked genes are passed from mothers to their sons. X-inked recessive inher-itance is exemplified in the transmission pattern of hemophilia A, a clotting disorder passed from carrier mothers to affected sons at a risk of 50%. The affected son can inherit either the mother’s normal allele or the mutant allele. A daughter has a 50% chance of inher-ited the hemophilia allele, and then becoming a car-rier (like her mother), as well as a 50% chance of not inheriting the allele.

If a father is affected and a mother is a carrier, their daughter could inherit an X-linked recessive dis-order. She would inherit one affected X chromosome from each parent. Unless having a biochemical test, the mother would not know she is a carrier of the X-linked recessive trait unless she had an affected son. When X-linked recessive traits seriously affect health, the affected male may not be well enough to father children. X-linked recessive conditions that are almost as common in females as males usually have mild phe-notypes. The mild X-linked trait of color blindness is an example. Men with this condition are just as likely to father children as men with normal color vision.

Dominant disease-causing alleles on the X chro-mosome are rare. Males are usually affected more severely, since females have a second X chromosome to offer some protection. The condition called incon-tinentia pigmenti causes females to have pigmented swirls of skin melanin, and may also have thin hair, abnormal teeth, visual problems, and seizures. How-ever, males with this condition have such severe effects that they usually die during gestation.

Gender Effects on Phenotype

Because of differences between genders, some auto-somal traits are expressed differently in males and females. A sex-limited trait affects a function or struc-ture that is present only in males or only in females. It involves genes that are X-linked or autosomal. Exam-ples of sex-limited traits include breast size and beard growth. Females do not grow beards because they do not manufacture enough hormones needed for facial hair growth. However, a mother can pass genes to her sons that specify heavy beard growth.

In sex-linked inheritance, an allele will be dom-inant in one sex but recessive in the other sex. This, too, involves genes that are X-linked or autosomal. The difference in expression shows hormonal dif-ferences between males and females. An example is the gene for hair growth pattern, which has two alleles. One allele produces hair all over the head and another allele causes pattern baldness. The baldness allele is dominant in males, yet recessive in females. This is why more males lose their hair than women.

A heterozygous male is bald, but a heterozygous female is not. For a women to go bald, she must have two mutant alleles.

In genomic imprinting, the expression of a disor-der is different, based on which parent transmits the gene or chromosome that causes the disorder. Only about 1% of human genes exhibit genomic imprint-ing. There may be differences in the severity of a phe-notype, the age at which it begins, and even in the types of symptoms. Genomic imprinting is physically based on methyl groups covering the gene from one of the parents and blocking it from being transcribed and translated.


Chromosome Disorders

An excess or deficit of genes is linked to deviations from the normal amount of 46 human chromosomes. Symptoms may also be linked to chromosome rear-rangement. There may be an inversion of one section of a chromosome or parts of two nonhomologous chromosomes may be exchanged. Rearrangement can disrupt vital genes or cause unbalanced gametes with too much or insufficient genetic material. When there are abnormal amounts of chromosomes, this can involve single chromosomes or entire sets.

Polyploidy

Polyploidy is when there is an entire extra set of chro-mosomes. It is the most serious type of chromosome disorder. It occurs from formation of a diploid gamete instead of a normal haploid gamete. When a haploid sperm fertilizes a diploid egg, the fertilized egg will be triploid, meaning it will have three copies of each chromosome. Usually, a polyploid embryo or fetus will stop development before being born. However, a poly-ploidy infant may survive for a few days after birth, having severe abnormalities. Various human organs have a few polyploid cells, but usually there is no seri-ous health effects. An example is the liver cells, which can have four or even eight sets of chromosomes.

Aneuploidy

Normal numbers of chromosomes are referred to as euploid. Cells that are missing a chromosome or have an extra chromosome are called aneuploidy. It results from an error in meiosis known as nondisjunction. Normally, pairs of homologous chromosomes separate during meiosis. Each resulting gamete has only one member of each pair. In nondisjunction, one chro-mosome pair does not separate, either in the first or second meiotic division. A sperm or egg with two cop-ies of a certain ­chromosome or no copies is ­produced instead of the normal one copy. When the gamete fuses with its mate during fertilization, the zygote will have either 47 or 45 chromosomes instead of 46.

Aneuploidy causes different symptoms based on the missing or extra chromosome. Autosomal aneu-ploidy often causes intellectual insufficiency, since many genes affect brain function. Sex chromosome aneuploidy is not as severe. Additional genetic material is less problematic than missing genetic material. Most children born with the wrong number of chromosomes have an extra one, which constitutes trisomy. When there is a missing chromosome, it is called monosomy.

Traditionally, aneuploidy conditions were named for the individuals who identified them. Today, they are designated by their affected chromosome. For example, Down syndrome is also referred to as ­trisomy 21, since a distinct set of its symptoms are linked to this chromosome. It is the most common autoso-mal aneuploidy. It can also develop when one copy of chromosome 21 exchanges parts with a different chromosome in a gamete and the fertilized ovum receives too much chromosome 21 material. This chromosomal abnormality is called a translocation. It is important to identify if the affected person has trisomy 21 or translocation Down syndrome, since the likelihood of trisomy 21 recurring in a sibling is only about one in 100, but translocation Down syndrome recurs in much greater numbers.

The next most common autosomal aneuploids are trisomies 13 and 18. They usually result in miscarriage of a fetus. An infant who has trisomy 13 will have an underdeveloped face, additional fingers and toes that are fused, small adrenal glands, heart defects, and a cleft lip or cleft palate. Trisomy 18 causes similar prob-lems as well as abnormal positioning of the fingers plus extra abdominal skin flaps. Trisomies of other autosomes do not develop beyond embryosis.

Sex chromosome aneuploids are less severe. XO syndrome is also called Turner syndrome, and affects only one in 2,000 newborn females, representing just 1% of XO conceptions. Delayed sexual development is often the only symptom. Though infertility is usu-ally present, hormone supplements help affected indi-viduals to live fairly normal lives. Nearly one of every 1,000–2,000 females has an extra X chromosome in their body cells. This condition is called triplo-X and usually causes menstrual irregularities and excessively tall height. When a male has an extra X chromosome, he may also have Klinefelter or XXY syndrome. Similar to Xo females, many XXY males usually do not realize their chromosomal makeup until developing fertility problems, resulting in testing. Related characteristics­ include underdevelopment of the testes and prostate glands and lack of pubic or facial hair. There may be excessive breast tissue growth, lengthening of limbs, and larger than normal hands and feet. About one of every 500–2,000 male births are affected by XXY syndrome.

One male of every 1,000 has an extra Y chromo-some. This is called XYY or Jacobs syndrome. There has been no occurrence of a fertilized ovum with one Y chromosome and no X chromosome. It is believed that when a zygote lacks an X chromosome, there is so little genetic material that only a few cell divisions will be possible, if the cells can divide at all.

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