© 2012 Pearson Education, Inc. Sexual Reproduction Meiosis Meiosis converts the diploid number (2n) of chromosomes to the haploid number (n) Produces genetic variation

If all reproduction mitosis, all be genetically identical except for the occasional mutation Life reproduced this way for the first 2 billion years and some still reproduce this way today Then about 1.5 to 2 billion years ago, cells began to reproduce genetically variable offspring through sexual reproduction

© 2012 Pearson Education, Inc. Figure 2.9

Sexual Reproduction and Genetic Variation Meiosis: the production of haploid gametes Fertilization: the fusion of haploid gametes Genetic variation: consequences of meiosis

Meiosis Interphase: DNA synthesis and chromosome replication phase Meiosis I: separation of homologous chromosome pairs, and reduction of the chromosome number by half Meiosis II: separation of sister chromatids, also known as equational division

© 2012 Pearson Education, Inc. Meiosis I and II each have prophase, metaphase, anaphase, and telophase stages

Meiosis I Prophase I Synapsis: close pairing of homologous chromosome Tetrad: closely associated four sister chromatids of two homologous chromosomes Crossing over: crossing over of chromosome segments from the sister chromatid of one chromosome to the sister chromatid of the other synapsed chromsome―exchange of genetic information, the first mechanism of generating genetic variation in newly formed gametes

Meiosis I Metaphase I: random alignment of homologous pairs of chromosomes along the metaphase plate Anaphase I: separation of homologous chromosome pairs, and the random distribution of chromosomes into two newly divided cells―second mechanism of generating genetics variation in the newly formed gametes Telophase I Interkinesis

Meiosis II Prophase II Metaphase II Anaphase II Telophase II

Consequences of Meiosis and Genetic Variation Four cells are produced from each original cell. Chromosome number in each new cell is reduced by half. The new cells are haploid. Newly formed cells from meiosis are genetically different from one another and from the parental cell.

Watch Crash Course Video on Meiosis

– What happens when errors occur in meiosis? – Such mistakes can result in genetic abnormalities that range from mild to fatal. When Meiosis Goes Awry

How Do Errors in Chromosome Number Affect Humans? The incorrect separation of chromosomes or chromatids in meiosis is known as nondisjunction – Nondisjunction causes gametes to have too many and too few chromosomes – Most embryos that arise from fusion of gametes with abnormal chromosome numbers spontaneously abort, but some survive to birth and beyond – Nondisjunction can occur in meiosis I or meiosis II

Meiosis I Abnormal gametes Gametes Nondisjunction: Pair of homologous chromosomes fails to separate. NONDISJUNCTION IN MEIOSIS I Number of chromosomes Meiosis II Nondisjunction: Pair of sister chromatids fails to separate. Abnormal gametes Normal gametes n n n  1 n – 1 n  1 NONDISJUNCTION IN MEIOSIS II n – 1 Figure

Abnormal egg cell with extra chromosome Normal sperm cell n  1 n (normal) Abnormal zygote with extra chromosome 2n  1 Figure 8.21

How Do Errors in Chromosome Number Affect Humans? Some genetic disorders are caused by abnormal numbers of autosomes – Nondisjunction of autosomes can occur during meiosis in the father or mother, resulting in eggs or sperm that are missing an autosome or that have two copies of an autosome Fusion of these gametes with a normal sperm or egg results in a zygote with one or three copies of the affected autosome Single-copy autosome embryos usually abort very early in development Embryos with three copies of an autosome (trisomy) also usually spontaneously abort; however, a small fraction of embryos with three copies of chromosomes 13, 18, or 21 survive to birth

How Do Errors in Chromosome Number Affect Humans? Some genetic disorders are caused by abnormal numbers of autosomes In trisomy 21 (Down syndrome), afflicted individuals have three copies of chromosome 21 Down syndrome includes several distinctive physical characteristics, including weak muscle tone, a small mouth held partially open because it cannot accommodate the tongue, and distinctively shaped eyelids Down syndrome is also characterized by low resistance to infectious diseases, heart malformations, and varying degrees of mental retardation, often severe Affects about one out of every 700 children

Chromosome 21 LM Figure 8.22

– The incidence of Down Syndrome increases with the age of the mother.

Age of mother Infants with Down syndrome (per 1,000 births) Figure 8.23

Edward’s Syndrome – Trisomy 18

Patau Syndrome – Trisomy 13

Nondisjunction in Sex Chromosomes Some genetic disorders are caused by abnormal numbers of sex chromosomes – Nondisjunction of sex chromosomes in males or females produces abnormal numbers of X and Y chromosomes – Nondisjunction of sex chromosomes in males produces sperm with either no sex chromosomes (called “O” sperm), or two sex chromosomes (sperm may be XX, YY, or XY) – Nondisjunction of sex chromosomes in females can produce eggs that are O or XX eggs instead of eggs with one X chromosome The most common abnormalities are XO, XXX, XXY, and XYY – Some sex chromosome abnormalities allow affected individuals to survive The genes on the X chromosome are so essential to survival, any embryo without at least one X chromosome spontaneously aborts very early in development

Table 10-2

– Turner syndrome (XO) occurs in females with only one X chromosome At puberty, hormone deficiencies prevent XO females from menstruating or developing secondary sexual characteristics Hormone treatment promotes physical development, but because affected women lack mature eggs, they remain infertile Additional symptoms include short stature, folds of skin around the neck, and increased risk of cardiovascular disease, kidney defects, and hearing loss Because they have only one X chromosome, women with Turner’s syndrome are more susceptible to recessive disorders such as red-green color blindness and hemophilia

– Men with Klinefelter syndrome (XXY) have an extra X chromosome Most afflicted males show no symptoms, although some may show mixed secondary sexual characteristics, including partial breast development, broadening of the hips, and small testes XXY men are often infertile because of low sperm count but are not impotent

– Trisomy X (XXX) results in a fertile “normal” woman with an extra X chromosome Most affected women show no abnormal symptoms There is an increased chance of learning disabilities and a tendency toward tallness associated with trisomy X By some unknown mechanism that prevents an extra X chromosome from being included in their eggs, women with trisomy X bear normal XX and XY children

Males with Jacob syndrome (XYY) have an extra Y chromosome (XYY) – Men with this malady have high levels of testosterone, tend to develop severe acne, and may be exceptionally tall