Patterns of Heredity
1. Inheritance and Mendel's Work
- Predicting Patterns of Heredity
- Meiosis
Parents and offspring
Some traits are acquired, not inherited. An acquired trait is developed during your life. Learned behaviors are one type of acquired trait.
Some acquired traits are not learned but result from interaction with the environment.
Genes are on chromosome pairs
Inherited traits are controlled by genes.
A gene is a unit of heredity that occupies a specific location on a chromosome and codes for a particular product. Heredity is the passing of genes from parents to offspring.
Individuals inherit their genes from their parents. The genes code for the expression of traits.
organism does not inherit the traits themselves from its parents. It inherits the genes that code for the traits it has.
Most traits are not coded for by just one gene.
Chromosome Pairs
In most eukaryotes, cells contain pairs of chromosomes, with one chromosome of each pair coming from each parent. The chromosomes in a pair are called homologs.
The illustration shows how a gene is a specific location on a chromosome. Each homolog has an allele, or alternate form, of the same gene. One allele is inherited from each parent. 
Human Chromosomes
Humans have 23 pairs, for a total of 46 chromosomes. Scientists refer to chromosomes by their number.
Human chromosomes are numbered 1–22; the 23rd pair is the sex chromosomes.
In humans, the sex chromosomes are called the X-chromosome and the Y-chromosome
In addition to determining the gender of an offspring, the X- and Y-chromosomes contain important genes, just as the other numbered chromosomes do.
Gregor Mendel's important discoveries about heredity
First investigating heredity were performed by a monk named Gregor Mendel, who lived in Austria during the mid-1800s.
He began investigating the inheritance of traits among the pea plants in the monastery's garden.
He worked with seven different traits: plant height, flower and pod position, seed shape, seed color, pod shape, pod color, and flower color. He studied each trait separately.
Mendel’s Conclusions
Mendel realized that each plant must have two “factors” for each possible trait—one factor from each parent.
Later experiments allowed Mendel to draw a number of other conclusions about how these factors are distributed. Since the mid-1800s, Mendel's experiments and conclusions have been the basis for most of the scientific thought about heredity. Those things he called “factors” are what we now call genes and alleles.
Punnett squares
A Punnett square illustrates how the parents' alleles might combine in offspring.
Each parent has two alleles for a particular gene. An offspring receives one allele from each parent.
probability of outcomes
A ratio compares, or shows the relationship between, two quantities.
Another way of expressing probability is as a percentage—a ratio that states the number of times a particular outcome might happen out of a hundred chances.
Calculating Probability
The ratios derived from a Punnett square tell you the probability that any one offspring will get certain genes and express a certain trait.
In humans, females have two X-chromosomes (XX), and males have an X- and a Y-chromosome (XY).
A Punnett square shows how the parents' alleles may be passed on to potential offspring.
A ratio is usually written 4:4 and read as “four to four.” This can be interpreted as “four out of four.”
The Punnett square showed that four out of four offspring will express the dominant gene for regular height.
The Punnett square of X- and Y-chromosomes shows the possible sexes of human offspring.
Unlike the guinea pig Punnett square, this one shows only two possible outcomes, XX and XY.
The diagram also shows how to find the percentage chance that a potential offspring will be female
One of the cells, an egg, contains genetic information from the mother. The other cell, a sperm, contains genetic information from the father. The two cells combine into a completely new cell, which becomes the offspring
Is necessary for sexual reproduction
Most human cells, contain 46 chromosomes—the full number of chromosomes that is normal for a human being.
Any cell that contains the full number of chromosomes (two sets) for a species is a 2n cell, also called a diploid cell.
Gametes are cells that contain half the usual number of chromosomes—one chromosome from each pair.
Fertilization
During sexual reproduction, two gametes combine to become a 2n cell that can grow into a new offspring.
Fertilization is the process that takes place when a sperm and an egg combine to form one new cell.
The diagram shows what happens to the chromosomes in gametes during fertilization
Gametes are formed by a special type of cell division called meiosis.
During meiosis, a single cell goes through two cell divisions—meiosis I and meiosis II.
Meiosis takes place only in the reproductive tissues of an organism.
Meiosis I
there are four steps in meiosis I:
Prophase I
Metaphase I
Anaphase I
Telophase I and Cytokinesis
The duplicated chromosomes pair up with their partners.
The chromosome pairs line up along the center of the cell.
The two copies of one homolog are pulled apart from the two copies of the other homolog.
A new cell membrane forms at the center of the cell, dividing the parent cell into two daughter cells.
Meiosis II
During meiosis I, two daughter cells are formed.
Both of these cells divide during meiosis II, to produce a total of four daughter cells.
The four steps in meiosis II are :
Prophase II
Metaphase II
Telophase II and Cytokinesis
Anaphase II
In each daughter cell, there are two copies each of n chromosomes. The copies are attached together.
Each duplicated chromosome lines up separately along each cell's center.
The two attached copies of each chromosome separate and are pulled to opposite poles in each cell.
new cell membrane forms in the center of each cell, as each cell divides into two 1n daughter cells, producing a total of four 1n cells.
Meiosis and mitosis differ
meiosis and mitosis are similar in many ways. However, they also have several very important differences.
Only cells that are to become gametes go through meiosis. All other cells divide by mitosis.
A cell that divides by meiosis goes through two cell divisions, but the chromosomes are not copied before the second division. In mitosis, the chromosomes are always copied before division.
Daughter cells produced by meiosis, which are haploid (1n), contain only half of the genetic material of the parent cell
Daughter cells produced by mitosis, which are diploid (2n), contain exactly the same genetic material as the parent