heredity

Heredity……. The genes for certain traits are passed down in families from parents to children. This has been known for thousands of years--even in Biblical times--and has allowed farmers to breed better crops and animals. For example, parents with black hair will likely give birth to children with black hair, just as parents with long noses will have kids with long noses. Once in awhile, though, this doesn’t work and parents with black hair will give birth to a blond. This discrepancy can be explained by the principle of segregation, first noted by Austrian monk Gregor Mendel over 100 years ago. The principle has three parts:

1. Hereditary traits are determined by specific genes. 2. Individuals carry two genes for each trait, one from the mother’s egg and one from the father’s sperm. 3. When an individual reproduces, the two genes split up (segregate) and end up in separate gametes. The principle of segregation applies to all organisms, including humans. 1. Hereditary traits are determined by specific genes. Within the DNA molecule, genes exist that specify a certain, single characteristic; there is a gene for height, a gene for weight, and a gene for eye color, etc. Variations of the gene relating to the same trait are called alleles. 2. Individuals carry two genes for each trait, one from the mother’s egg and one from the father’s sperm. One of these two genes is dominant over the other. The dominant allele will mask the other, called the recessive allele. For example, if the father gives a tall allele of the height gene, and the mother gives a short allele, the offspring will be tall. This is because tall is dominant and short is recessive. The British mathematician/biologist R.C. Punnett devised a method of picturing this concept on a graph called a Punnett Square. Punnett Squares graph the father’s genotype (the genetic information concerned with a specific trait: for example, two alleles for tall, or two for short, or one for each) crossed with the mother’s. Punnett Squares show the probability of having children who have a certain trait.

• Dominant alleles are shown by a capital letter.
• Recessive alleles are shown by the lowercase of the same letter.

t t T Tt Tt t tt tt

This graph is a cross between a mother who is a hybrid or heterozygous for tall (meaning she has one allele (T) for tallness and one (t) for shortness). Physically she is tall because T is dominant and masks the shortness genes from the father. Half of their offspring will therefore be short (tt) and half will be tall hybrids (Tt; a pure tall offspring would be TT). This means that the parents have a 2/4 or 50% chance of having tall children and a 2/4 or 50% chance of having short children. This is a 1:1 ratio.

More examples:

T T T TT TT t Tt Tt

All kids will be tall: 4:0 ratio 50% will be pure 50% will be hybrids

T t T TT Tt t Tt tt

3 of 4 kids will be tall: 3:1 ratio 75% chance of being tall 25% chance of being pure tall 50% chance of being hybrid tall 25% chance of being short

When only one trait is a Punnett Square is graphed, it is called a monohybrid cross. But when two or more traits are graphed, it’s called a dihybrid cross. This illustrates the law of Independent Assortment, meaning that one trait doesn’t affect another. In other words, having red hair has nothing to do with also having bad eyesight. The genes are independent of each other.

	TB Tb tB tb TB TTBB TTBb TtBB TtBb Tb TTBb Ttbb TtBb Ttbb tB TtBB TtBb ttBB ttBb tb TtBb Ttbb ttBb ttbb

Sometimes two genes will be co-dominant--that is, neither masks the other. In this case, both genes will show. An example is skin color: the child of dark-skinned and fair-skinned parents will be a mixture of the two. Breeding red geraniums with white geraniums gives you pink flowers.

3. When an individual reproduces, the two genes split up (segregate) and end up in different gametes. This is explained by the process called meiosis. Meiosis is like mitosis (normal cell division), but instead produces sex cells (gametes: sperm and egg). Sex cells have only 23 chromosomes (called a haploid, meaning “one set”), instead of 46 (called a diploid, meaning “two sets”) so that when fertilization occurs, a new cell with 46 chromosomes will form. For example, when a sperm with 23 chromosomes unites with an egg with 23 chromosomes, the cell they form will have 46.

In meiosis, the cell divides normally (as in mitosis) after copying its chromosomes. The chromosomes also undergo crossing-over. When the chromosomes pair up, sometimes they will switch genetic data. This ensures that the genes from both parents will be present. Immediately after dividing, it divides again, this time without copying the chromosomes. This creates four sex cells, where only one existed before, each with only 23 chromosomes.