Sexual Reproduction Meiosis

The problem with sexual reproduction Each human cell has 46 chromosomes. If every egg and every sperm had 46 chromosomes, how many would be found in the fertilized egg?

The problem with sexual reproduction 92!

The problem with sexual reproduction 92! But the fertilized egg has 46 chromosomes, 23 from each parent. So what happened?

The problem with sexual reproduction The number of chromosomes is reduced by half. This is a special type of cell division called meiosis.

The problem with sexual reproduction Meiosis is the process by which the number of chromosomes is reduced by half. During this process the homologous chromosomes are separated.

The problem with sexual reproduction Meiosis occurs in those tissues that give rise to gametes (eggs or sperm). Gametogenesis (the formation of gametes) occurs only in the gonads (testes or ovaries).

The problem with sexual reproduction The typical human body cell contains 46 chromosomes. This is called the diploid or 2n state.

The problem with sexual reproduction After meiosis, the homologous pairs are separated and the chromosome number is now haploid or 1n.

The problem with sexual reproduction Homologous chromosomes contain similar types of genetic information.

The problem with sexual reproduction Homologous chromosomes contain similar types of genetic information. Each member of the pair comes from one parent.

The problem with sexual reproduction The human has 23 pairs of homologous chromosomes. During meiosis the pairs separate forming gametes with only one part of the pair.

Meiosis Meiosis looks like two mitotic divisions. The major difference is that 4 cells instead of two are formed and the chromosome number is reduced by half.

Important Steps In the first metaphase in meiosis, the homologous chromosomes pair up.

Important Steps The homologous chromosomes pairing is called a tetrad.

Important Steps During this phase, crossing over can occur. Crossing over is where chromatids can exchange pieces of genetic information.

Important Steps This results in more genetic variation.

Important Steps In the second metaphase of meiosis, the paired chromatids split. This looks very much like metaphase in mitosis.

Mitosis vs Meiosis

The problem with sexual reproduction Meiosis can go wrong. During Metaphase I or 2, the chromosomes may not separate and this results in gametes having the wrong number of chromosomes.

The problem with sexual reproduction Nondisjunction is when the chromosomes fail to separate properly during meiosis. In the human this can result in eggs or sperm having the incorrect number of chromosomes.

The problem with sexual reproduction Common syndromes caused by nondisjunction include: 1 Downs Syndrome 2 Edwards Syndrome 3 Kleinfelters Syndrome 4 Turners Syndrome

Diagnosis A karyotype is a pictorial presentation of an individual’s chromosomes, taken from photographs taken through a microscope. The chromosomes can then be classified and paired up to find any mistakes.

Diagnosis Chromosomes are divided into two types: Autosomes which are in homologous pairs and the Sex Chromosomes. These can be paired for the female XX or hemizygous for the male XY

Diagnosis The chromosomes are arranged in a numeric sequence that aligns the chromosomes from largest to smallest.

Diagnosis The karyotype shows the matching homologous chromosomes. There are 22 pairs of autosomes and 1 set of sex chromosomes.

Diagnosis For example, Chromosomal pair 1, one is from the mother and the other from the father. This individual is 46, XX

Diagnosis Karyotypes can also be used to determine the sex of the infant. The 23rd pair are the sex chromosomes.

Diagnosis If this pair is a XX, the infant is a female.

Diagnosis If this pair is a XY, the infant is a male. 46,XY

Preparation of a Karyotype For prenatal diagnosis, an amniocentesis is performed. This takes a sample of amniotic fluid which contains cells from the infant.

Preparation of a Karyotype Ultrasound Image Amniocentesis

Preparation of a Karyotype

Preparation of a Karyotype The karyotype can be analyzed for any chromosomal abnormalities.

Syndrome 1

Syndrome 1 What is the sex? Is there any abnormality?

Syndrome 1 Down’s Syndrome This is a male with trisomy 21 (3 chromosome # 21) Downs syndrome is the most common nondisjunctional condition.

Syndrome 1 Down’s Syndrome Signs and symptoms include: Flatten face Makes the tongue protrude Loose joints & poor muscle tone Upward slant to the eyes Short stature Mental retardation (varies)

Syndrome 2

Syndrome 2 What is the sex? Is there any abnormality?

Syndrome 2 Kleinfelter’s Syndrome Signs and symptoms : These individuals are males with a XXY genotype Symptoms are not seen until puberty with a failure of the secondary sex characteristics to develop

Syndrome 2 Kleinfelter’s Syndrome Signs and symptoms : The extra X chromosome causes the testicles not develop adequately. Some have learning disabilities.

Syndrome 2 Kleinfelter’s Syndrome Signs and symptoms : Estrogen and growth hormone are given to allow normal development.

Syndrome 3

Syndrome 3 What is the sex? Is there any abnormality?

Syndrome 3 Turner’s Syndrome Signs and symptoms : These individuals are females with a X O genotype Symptoms are not seen until puberty with a failure of the secondary sex characteristics to develop. Intelligence is normal

Syndrome 3 Turner’s Syndrome Signs and symptoms : The missing X chromosome causes the ovaries not to develop. Some have learning disabilities.

Other Syndromes Patau, an extra chromosome 13 Edwards, an extra chromosome 18 Super females extra X chromosome Super males extra Y chromosome