Human Heredity, Part 1

What We Should Already Know: In order to reproduce, organisms must pass on their genes to the next generation. Genes, which are made of DNA, are stored in large, tangled structures called chromosomes.

What We Should Already Know: Humans, like other animals, are diploid: that is, their cells contain two copies of each chromosome.

What We Should Already Know: If they were like bacteria, human beings would just copy all of their chromosomes and their offspring would be clones (genetically identical copies)

What We Should Already Know: But human beings are not like bacteria! Instead, humans (like all other animals) reproduce sexually: that is, they exchange genes with other members of their species, producing offspring which are genetically unique.

What We Should Already Know: In order to do this, however, species which practice sexual reproduction must manufacture two different kinds of haploid cells, called gametes, that each have only one copy of each chromosome.

What We Should Already Know: One type of gamete is typically small and manufactured in large quantities. Individuals that make this kind of gamete (called sperm) are known as males, and adult males manufacture millions of gametes every day!

What We Should Already Know: The other type of gamete is typically large, complex and manufactured in tiny quantities. These cells, called eggs, are made by females. If these cells fuse with a sperm, the fertilized cell (or zygote) can develop into a new human being.

Chromosomes can be isolated under a microscope and photographed. Human Chromosomes Chromosomes can be isolated under a microscope and photographed.

Human Chromosomes The separate images can be assembled into a single picture that shows the entire set of chromosomes, called a karyotype.

Human Chromosomes Here we see a set of chromosomes. They are arranged in pairs of similar size and shape, called homologous chromosomes.

How many pairs in this karyotype? How many total chromosomes? Human Chromosomes How many pairs in this karyotype? How many total chromosomes?

One pair, position 23, shows two chromosomes Human Chromosomes One pair, position 23, shows two chromosomes that do not have the same size and shape.

Human Chromosomes The larger one is often called the X chromosome, while the smaller is referred to as the Y chromosome.

Human Chromosomes It is this pair that determines whether an individual is male or female, and so position 23 are often called the sex chromosomes.

Human Chromosomes Positions 1-22, which are not involved in sex determination, are classified as autosomes.

All genetically-normal human beings have Human Chromosomes All genetically-normal human beings have at least one copy of the X chromosome. . .

Human Chromosomes Females have two copies, and so they are said to be XX; males, on the other hand, are XY.

Since only males have Y, it is the father Human Chromosomes Since only males have Y, it is the father that determines the child’s sex.

Gametes (eggs and sperm) are produced in a version of cell division called meiosis. In the first part of meiosis, the homologous chromosomes seen in a normal karyotype are each copied, then come together in groups of four called tetrads.

They are so close that whole chunks of chromosome may be exchanged, a process called ‘crossing-over’. This process generates new combinations of genes and is one of the reasons that sexual reproduction leads to genetically-unique individuals.

Ordinarily, the tetrads separate, undergo two rounds of cell division right after another,producing four haploid daughter cells, each with just one copy of each chromosome,and it is these cells that will become either eggs and sperm.

But what if the chromosomes don’t properly separate?

This condition, called non-disjunction, will lead to daughter cells (and, eventually, gametes) that have the wrong number of chromosomes.

If one of these defective egg or sperm are fertilized, this can lead to an individual with a chromosomal disorder . .

Meiosis Gone Wrong: Down's Syndrome

Meiosis Gone Wrong: Down's Syndrome

Meiosis Gone Wrong: Down's Syndrome

Meiosis Gone Wrong: Down's Syndrome

Meiosis Gone Wrong: Down's Syndrome is a chromosomal disorder caused by non-disjunction

Meiosis Gone Wrong: Down's Syndrome is a chromosomal disorder caused by non-disjunction is caused by trisomy (having 3 copies) of chromosome 21

Meiosis Gone Wrong: Down's Syndrome

Meiosis Gone Wrong: Down's Syndrome is a chromosomal disorder caused by non-disjunction is caused by trisomy (having 3 copies) of chromosome 21

Meiosis Gone Wrong: Down's Syndrome leads to mental retardation, birth defects and other health problems occurs in about 1 out of every 800 live births in the U.S.

Down’s Syndrome: is more likely to affect the babies of older mothers, whose eggs have accumulated more mutations.

Down’s Syndrome: is a condition that affects the Meiosis Gone Wrong: Down's Syndrome Down’s Syndrome: is a condition that affects the lives of human beings, who despite their limitations can still lead productive, happy lives

Sex Chromosome Disorders: typically lead either to death or sterility no individual can survive without an X chromosome, while the wrong number of either X or Y usually makes the individual sterile

Sex Chromosome Disorders: typically lead either to death or sterility no individual can survive without an X chromosome, while the wrong number of either X or Y usually makes the individual sterile Sex Chromosome Disorders:

Sex Chromosome Disorders: Turner’s Syndrome produces females who often appear and act normal, but who never develop ovaries capable of making eggs; they are sterile. Sex Chromosome Disorders:

Turner’s Syndrome produces females who often appear and act normal, but who never develop ovaries capable of making eggs; they are sterile.

Sex Chromosome Disorders: Kleinfelter’s Syndrome produces males whose appearance and behavior as adults is often unusual: a relatively high number of these individuals are institutionalized. Sex Chromosome Disorders:

Kleinfelter’s Syndrome produces males whose appearance and behavior as adults is often unusual: a relatively high number of these individuals are institutionalized.

Sex Chromosome Disorders: Students often ask whether or not any individual human beings are truly hermaphroditic: that is, naturally having the appearance or function of both sexes? The answer is no!

However, for a variety of reasons, some Sex Chromosome Disorders: However, for a variety of reasons, some individuals appear to suffer from gender identity problems, and this may have a genetic basis. Affected individuals may choose surgery to alter their appearance, and some (not all) experience an improvement in the quality of their lives as a result. As with Down’s Syndrome, those affected by conditions which affect their identity or appearance deserve to be treated as human beings.