Genetics and Heredity How DNA Leads to eye color, height and dimples!

Genetics and Heredity

How DNA Leads to eye color, height and dimples!

Mendel and His Peas

Who Was Gregor Mendel?

• Gregor Mendel was born in 1822 in Heinzendorf, Austria.

• At age 21, Mendel entered a monastery. He performed many scientific experiments in the monastery garden.

• Mendel discovered the principles of heredity, the passing of traits from parents to offspring.

Unraveling the Mystery

• Mendel used garden pea plants for his experiments.

• Self-Pollinating Peas have both male and female reproductive structures. So, pollen from one flower can fertilize the ovule of the same flower.

• When a true-breeding plant self pollinates, all of the offspring will have the same trait as the parent.

Mendel and His Peas

Unraveling the Mystery, continued

• Pea plants can also cross-pollinate. Pollen from one plant fertilizes the ovule of a flower on a different plant.

• The image below shows cross-pollination and self-pollination.

Mendel and His Peas


• Characteristics and Traits of Pea Plants Mendel studied only one pea characteristic at a time. A characteristic is a feature that has different forms in a population.

• Different forms of a characteristic are called traits.

Mendel and His Peas


• Mix and Match Mendel was careful to use plants that were true breeding for each of the traits he was studying. By doing so, he would know what to expect if his plants were to self-pollinate.

Mendel and His Peas

Mendel’s First Experiments• Mendel crossed pea plants to study seven

different characteristics.

• Mendel got similar results for each cross. One trait was always present in the first generation, and the other trait seemed to disappear.

• Mendel called the trait that appeared the dominant trait. The trait that seemed to fade into the background was called the recessive trait.

Mendel and His Peas

Mendel’s SecondExperiments

• To find out more about recessive traits, Mendel allowed the first-generation plants to self-pollinate.

• In each case some of the second-generation plants had the recessive trait.

Mendel and His Peas

Mendel’s Second Experiments, continued

• Ratios in Mendel’s Experiments The recessive trait did not show up as often as the dominant trait.

• Mendel decided to figure out the ratio of dominant traits to recessive traits.

Mendel and His Peas


In all cases the ratio was about 3:1 dominant : recessive.

Mendel and His Peas


• Gregor Mendel – Gone But Not Forgotten Mendel realized that his results could be explained only if each plant had two sets of instructions for each characteristic.

• Mendel’s work opened the door to modern genetics.

Mendel and His Peas

A Great Idea

• Mendel knew that there must be two sets of instructions for each characteristic.

• The instructions for an inherited trait are called genes.

• The different forms (often dominant and recessive) of a gene are alleles.

Traits and Inheritance

A Great Idea, continued

• Dominance occurs when certain alleles mask the expression of other alleles.

• A recessive trait or allele is expressed only when two recessive alleles for the same characteristic are inherited.

• Phenotype An organism’s appearance is known as its phenotype. Genes affect the phenotype.


• Genotype The combination of inherited alleles together form an organism’s genotype.

• A plant with two dominant or two recessive alleles is said to be homozygous.

• A plant that has the genotype Pp is said to be heterozygous.



• Punnett Squares are used to organize all the possible genotype combinations of offspring from particular parents.




What Are the Chances?

• Probability is the mathematical chance that something will happen.

• Probability is most often written as a fraction of percentage.


• Genotype Probability To have white flowers, a pea plant must receive a p allele from each parent. Each offspring of a Pp Pp cross has a 50% chance of receiving either allele from either parent. So, the probability of inheriting two p alleles is 1/2 1/2, which equals 1/4, or 25%.


What Are the Chances?, continued

Gene Interactions and Variations

• Incomplete Dominance Researchers have found that sometimes one trait is not completely dominant over another.

• One Gene, Many Traits Sometimes one gene influences more than one trait.

• Many Genes, One Trait Some traits, such as the color of your skin, hair, and eyes, are the result of several genes acting together.


The Importance of Environment

• Genes aren’t the only influences on traits. A combination of things determine an individual’s characteristics.

• Your environment also influences how you grow.

• Lifestyle choices can also affect a person’s traits.


• In asexual reproduction, only one parent cell is needed. The structures inside the cell are copied, and then the parent cell divides, making two exact copies.

• This type of cell reproduction is called mitosis. Most of the cells in your body and most single-celled organisms reproduce this way.

Meiosis

Asexual Reproduction

Mitosis

Meiosis

• Advantages of Asexual Reproduction One advantage of asexual reproduction is that organisms can produce many offspring in a relatively short amount of time.

• An advantage for animals that reproduce asexually is that they do not have to use energy to find a mate.

Meiosis

Asexual Reproduction, continued

Sexual Reproduction

• In sexual reproduction, two parent cells (sex cells) join together to form offspring that are different from both parents.

• Chromosomes that carry the same sets of genes are called homologous chromosomes.

• Each sex cell has only one of the chromosomes from the homologous pair.

Meiosis

Sexual Reproduction, continued

• Advantages of Sexual Reproduction The combination of genetic information during sexual reproduction allows for variation among a population.

• The variation of genes allows a population to adapt to changes in the environment over time.

Meiosis


• Meiosis Sex cells are made during meiosis.

• Meiosis is a copying process that produces cells with half the usual number of chromosomes.

Meiosis

• Genes and Chromosomes Walter Sutton studied meiosis in sperm cells in grasshoppers.

• Using his observations and his knowledge of Mendel’s work, Sutton proposed that:

Genes are located on chromosomes.

Meiosis


The Steps of Meiosis

• During meiosis, chromosomes are copied once, and then the nucleus divides twice.

• The resulting sex cells (sperm and eggs) have half the number of chromosomes of a normal body cell.

Meiosis

Meiosis

Meiosis

Meiosis and Fertilization

• The next slide shows what happens to a pair of homologous chromosomes during meiosis and fertilization. The cross shown is between a plant that is homozygous for round seeds (dominant trait) and a plant that is homozygous for wrinkled seeds (recessive trait).

Meiosis

Meiosis

Meiosis and Fertilization, continued

• Sex Chromosomes carry genes that determine sex.

• Human females have two X chromosomes.

• Human males have one X chromosome and one Y chromosome.

Meiosis

Meiosis and Fertilization, continued

• Sex-Linked Disorders The genes for certain disorders, such as colorblindness, are carried on the X chromosome.

• Genetic Counseling Genetic counselors use pedigrees to trace traits through generations of a family. These diagrams can often predict if a person is a carrier of a hereditary disease.

• Selective Breeding In selective breeding, organisms with desirable characteristics are mated.

Meiosis

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Genetics and Heredity How DNA Leads to eye color, height and dimples!