Genetics

Genetics is the study of heredity. Heredity is when a parent passes down physical characteristics to their offspring. These different physical characteristics are known as traits.

Mendel’s Experiment Gregor Mendel’s experiments helped create the field of genetics. Mendel studied the characteristics of pea plants and wondered why they had different characteristics. What determined which characteristic the pea plant would exhibit?

Mendel’s Experiment Pea plants usually self-pollinate, meaning that they are the only parent to their offspring. Mendel cross-pollinated the pea plants by removing the pollen from one plant and brushing on the pollen from a second pea plant.

Mendel’s Experiment Mendel crossed pea plants that showed different traits. He used purebred pea plants to start his experiment. – A purebred organism is one that has exhibited the same trait for many generations.

Mendel’s Experiment The P generation – The P generation is the parent generation and they are the original plants. – In Mendel’s experiment his P generation had one purebred tall pea plant and one purebred short pea plant.

Mendel’s Experiment The F 1 Generation – The offspring of the P generation is known as the F 1 generation, or the first filial generation. The word filial comes from the Latin words for “daughter” and “son”. – In Mendel’s experiment, all of the offspring in the F 1 generation was tall, despite one parent being short. – The shortness trait seemed to have disappeared.

Mendel’s Experiment The F 2 Generation – Mendel allowed the F 1 generation to self- pollinate. – This new offspring Mendel called the F 2 generation. – Somehow, the shortness trait seemed to have reappeared. – About ¾ of the F 2 generation was tall, while ¼ was short.

Mendel’s Experiment Mendel also crossed pea plants with different contrasting traits. In each of Mendel’s crosses, only one trait appeared in the F 1 generation. However, each time, the “lost” trait appeared in about ¼ of the F 2 generation.

Alleles Today, scientists use the word gene for the factors that control a trait. They refer to the different forms of that trait as alleles.

Alleles An organism’s traits are controlled by the alleles it inherits from its parents. – It receives one allele from each. Some alleles are dominant. Some are recessive.

Alleles A dominate trait will always show up if an allele for it is present in an organism. A recessive trait only appears if both alleles are recessive.

Alleles If an organism has one of each type of allele it is considered a hybrid. If an organism has both a dominant and a recessive allele, it will only show the dominant trait. – For example, tall height was dominant, while short was recessive in the pea plants.

Alleles Scientists represent dominant alleles with capital letters and recessive alleles with lowercase letters.

Probability Probability is a number that describes how likely it is that an event will occur. When you have two options that are equally likely to occur, there is a 50% chance that either will happen.

Probability The laws of probability predict what is likely to occur, not what necessarily will occur. Because of this, the results of an event, such as flipping a coin, will not always be exactly 50/50. The more times you try (for example, flipping the coin) the closer to 50/50 your results will be.

Probability The results of the first flip of a coin does not affect the results of the next flip.

Punnett Squares Scientists use Punnett squares to help show how probability applies to genetics. Punnett squares show all the possible combinations of alleles that can happen after a genetic cross. – A genetic cross is when two parents produce an offspring and each pass down one allele.

Punnett Squares

You can use a Punnett square to calculate the probability that an offspring will have a certain combination of alleles. In a genetic cross, the allele that each parent will pass on to its offspring is based on probability.

Phenotypes and Genotypes An organism’s phenotype is its physical appearance, or visual traits. An organism’s genotype is its genetic makeup, or allele combinations.

Phenotypes and Genotypes Two organisms can show the same dominant phenotype, but have different genotypes. – One can have two dominant alleles. Example: RR – The other can have one dominant and one recessive allele. Example: Rr

Phenotype and Genotype An organism that has two identical alleles for a trait is said to be homozygous. (RR or rr) An organism that has two different alleles for a trait is said to be heterozygous. (Rr) We know that homo means “the same” and hetero means “different”.

Phenotype and Genotype Mendel used the term hybrid to describe heterozygous pea plants.

Codominance For all of the traits that Mendel studied, one allele was dominant while the other was recessive. This is not always the case.

Codominance Codominance is when the alleles are neither recessive nor dominant. As a result, when these alleles are combined, both show up in the offspring’s appearance.

The Chromosome Theory of Inheritance According to the chromosome theory of inheritance, genes are carried from the parents to their offspring on chromosomes.

The Chromosome Theory of Inheritance Sex cells (sperm and egg) have exactly half the number of chromosomes found in an organism’s body cells. When the sperm and the egg join during fertilization, the offspring ends up with exactly the same number of chromosomes in its cells as each of its parents have in theirs.

The Chromosome Theory of Inheritance One allele for each gene comes from the organism’s female parent and the other allele comes from the male parent. These paired alleles were carried on paired chromosomes.

Meiosis Meiosis is the process by which the number of chromosomes is reduced by half to form sex cells.

Meiosis Before meiosis, every chromosome in the cell is copied. Then, these pairs line up in the center of the cell, separate, and move to opposite ends of the cell. The cell splits, creating two cells, each with half of the chromosomes.

Meiosis These chromosomes now move to the center of the new cells. The centromeres split, and the chromatids move to opposite sides of the cells. These cells split, creating sex cells with only one chromosome from the original pair.

Human Chromosomes Humans have 23 chromosome pairs (46 total chromosomes) in every cell. Chromosomes are made up of many genes joined together. Although you have 23 chromosome pairs, you have about 35,000 genes.

Human Chromosomes One chromosome in each chromosome pair comes from the mother and the other from the father. Each chromosome in the pair has the same genes, but the alleles for the gene in the two chromosomes can be different.