Each sperm cell contains half the father’s DNA. But it’s not identical from sperm to sperm because each man is a mixture of the genetic material from his parents, and each time a slightly different assortment of that full DNA set gets divided to go into a sperm.

The matched pairs of chromosomes in a diploid organism are called homologous chromosomes.

Mitosis. Interphase: Each chromosome undergoes replication, making an identical copy of itself.

Each identical individual rod in a chromosome is called a chromatid, while together the two chromatids are called chromosomes.

Mitosis

Chromosomes are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA).

Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body. Chromosomes are structures within cells that contain a person’s genes.

The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people.

There are 4 different bases in DNA: Guanine (G), Adenine (A), Cytosine (C) and Thymine (T). The sequence in which the bases occur is a code that contains information.

The bases go by the names of adenine, cytosine, thymine, and guanine, otherwise known as A, C, T, and G. DNA is a remarkably simple structure. It’s a polymer of four bases–A, C, T, and G–but it allows enormous complexity to be encoded by the pattern of those bases, one after another.

What kinds of gene variants are possible?

• Missense. A missense variant is a type of substitution in which the nucleotide change results in the replacement of one protein building block (amino acid) with another in the protein made from the gene.
• Nonsense.
• Insertion.
• Deletion.
• Duplication.
• Frameshift.
• Repeat expansion.

Point mutation, change within a gene in which one base pair in the DNA sequence is altered. Point mutations are frequently the result of mistakes made during DNA replication, although modification of DNA, such as through exposure to X-rays or to ultraviolet radiation, also can induce point mutations.

Sometimes gene mutations are beneficial, but most of them are detrimental. For example, if a point mutation occurs at a crucial position in a DNA sequence, the affected protein will lack biological activity, perhaps resulting in the death of a cell.

Codominance is a relationship between two versions of a gene. In codominance, however, neither allele is recessive and the phenotypes of both alleles are expressed.

When egg and sperm cells combine in fertilisation, they merge the two sets of chromosomes, ending up with 46 chromosomes in total. The maternal chromosomes from the egg cell and the paternal chromosomes from the sperm cell pair up. The resultant cell is called a zygote. The zygote will mature into an embryo .

Euploidy refers to the change in the complete set of chromosomes, i.e. loss or gain of the full set of chromosomes. Aneuploidy refers to the gain and loss of one or two chromosomes, e.g. monosomy (2n-1), trisomy (2n+1), nullisomy (2n-2).

Aneuploidy means an organism does not have the normal number of chromosomes. If there is 1 less chromosome, this is referred to as monosomic, abbreviated as 2n – 1. If there is 1 extra chromosome, this is called trisomic, abbreviated as 2n + 1. Examples of aneuploidy are trisomy 21 and XYY Klinefelter’s Syndrome.

{ Aneuploidy — Excess or Defficiency in a single chromosome. The cell(s) have the same number of copies of all the chromosome. Euploidy is an excess or deficiency for the entire complement of chromosomes. w Some cells in other organisms (liver, midgut, etc.)

Trisomy is the most common aneuploidy. In trisomy, there is an extra chromosome. A common trisomy is Down syndrome (trisomy 21). Other trisomies include Patau syndrome (trisomy 13) and Edwards syndrome (trisomy 18).

Listen to pronunciation. (AN-yoo-PLOY-dee) The occurrence of one or more extra or missing chromosomes leading to an unbalanced chromosome complement, or any chromosome number that is not an exact multiple of the haploid number (which is 23).

Aneuploidy is a chromosomal mutation in which there is one or more extra chromosomes, or one or more fewer chromosomes. Polyploidy is a chromosomal mutation in which a cell has entire extra sets of chromosomes.

Down Syndrome is probably the most well-known example of a chromosomal aneuploidy, caused by an extra copy of chromosome 21 known as trisomy 21. While a trisomy can occur with any chromosome, the condition is rarely viable.

Humans. True polyploidy rarely occurs in humans, although polyploid cells occur in highly differentiated tissue, such as liver parenchyma, heart muscle, placenta and in bone marrow. In some cases, survival past birth may be extended if there is mixoploidy with both a diploid and a triploid cell population present.

Down syndrome is caused by a random error in cell division that results in the presence of an extra copy of chromosome 21. The type of error is called nondisjunction (pronounced non-dis-JUHNGK-shuhn).

There are three types of Down syndrome:

• Trisomy 21. This is by far the most common type, where every cell in the body has three copies of chromosome 21 instead of two.
• Translocation Down syndrome. In this type, each cell has part of an extra chromosome 21, or an entirely extra one.
• Mosaic Down syndrome.

Most of the time, Down syndrome isn’t inherited. It’s caused by a mistake in cell division during early development of the fetus. Translocation Down syndrome can be passed from parent to child.

If the screening test shows that the chance of having a baby with Down’s syndrome, Edwards’ syndrome and Patau’s syndrome is lower than 1 in 150, this is a lower-chance result.

Many women are able to carry pregnancies after age 35 and beyond. However, there are certain risks — for both mother and baby — that tend to increase with maternal age. Infertility. It may take longer to get pregnant as you get closer to menopause.

Giving birth after 35 is risky. Pregnant women past the age of 35 have an increased risk of pregnancy complications, including gestational diabetes, preeclampsia (high blood pressure) and intrauterine growth restriction (causing premature delivery).

The test is usually done at 15 to 22 weeks of pregnancy. Integrated screening test. Your provider combines the results from the first trimester screening and the second trimester screening to better estimate the chances that your baby may have Down syndrome.

Just like any baby, infants who have Down syndrome will sometimes be fussy. The temperament of a baby with Down syndrome is not usually any different from the temperament of any other baby.

A–Z

Do Individuals with Down Syndrome Have Sexual Feelings? In the past, sexuality was not considered an issue for any people with Down syndrome because of the inaccurate belief that intellectual disability produced permanent childhood. In fact, all people with Down syndrome have sexual feelings and intimacy needs.

I.Q. scores for people with Down syndrome vary, with the average cognitive delays being mild to moderate, not severe. In fact, normal intelligence is possible. If a person with Down syndrome has difficulty with hearing, it can be misinterpreted as a problem with understanding.

Is 80-year-old British pensioner the world’s oldest living man with Down’s syndrome? A “cheeky chap” who defied all medical odds could be the world’s oldest living man with Down’s syndrome after celebrating his 80th birthday.

Heart and lung diseases are the leading causes of death for persons with Down syndrome. Pneumonia and infectious lung disease, congenital heart defect (CHD) and circulatory disease (vascular diseases not including CHD or ischaemic heart disease) account for ∼75% of all deaths in persons with Down syndrome.

Today the average lifespan of a person with Down syndrome is approximately 60 years. As recently as 1983, the average lifespan of a person with Down syndrome was 25 years.

Quick Facts The average life expectancy for a person who has Down’s syndrome is between 50 and 60 with a small number of people living into their seventies.