Genetics & Heredity 8th Grade Science

What is Genetics & Heredity? Genetics: The field of science that deals with heredity. Heredity: Passing on of characteristics from parents to offspring. From the Latin word hered, meaning “heir” Although the field of genetics is recent; people have been manipulating genes for thousands of years, for example... Selective Breeding (Pets, Livestock, Crops)

Genetics: Questions…. Why do offspring resemble their parents? (for example, hippos have baby hippos, turtles have baby turtles) vs.

Genetics: Questions…. Why do some individuals in a population differ?

Genetics: Questions…. How do species change over time?

What gene is responsible? How does it function? Genetics: Questions…. How are traits transmitted? (i.e., number of fingers on a hand, brown hair) What gene is responsible? How does it function?

It all comes down to your chromosomes Chromosomes are in the nucleus of every cell They carry the genetic information (DNA) passed on, generation to generation.

Chicks ?

Dudes

A human egg with many sperm trying to fertilize it (only one will get lucky!)

The Genetic Code is Universal The same type of DNA exists in every organism, just in a different order

The closer the order of the DNA between organisms, the closer the organisms are related in terms of evolution

Who was Gregor Mendel? A priest, born in 1822 Studied science, mathematics and statistics Worked on heredity in peas for 10+ years Discovered the principles of heredity “The father of modern genetics”

These are the seven characteristics, each having two contrasting traits, that Mendel identified in peas.

Heredity Terminology Characteristic: observable feature that can appear in more than one form (flower color) Trait: a variation that can exist for a characteristic (purple or white flowers)

Heredity Terminology Purebred: all offspring and its descendants have the same trait for a characteristic when they are cross pollinated (breed together) (used in Mendel’s experiments) Hybrid: results from the crossing of 2 purebred plants that have different traits (purple x white)

The Role of Fertilization Mendel decided to “cross” his stocks of true-breeding plants—he caused one plant to reproduce with another plant.

Genes and Alleles When doing genetic crosses, we call the original pair of plants the P, or parental, generation.

The Role of Fertilization To do this, he had to prevent self-pollination. He did so by cutting away the pollen-bearing male parts of a flower and then dusting the pollen from a different plant onto the female part of that flower, as shown in the figure.

The Role of Fertilization This process, known as cross-pollination, produces a plant that has two different parents. Cross-pollination allowed Mendel to breed plants with traits different from those of their parents and then study the results.

Genes and Alleles Their offspring are called the F1, or “first filial,” generation.

Genes and Alleles For each trait studied in Mendel’s experiments, all the offspring had the characteristics of only one of their parents, as shown in the table.

Genes and Alleles In each cross, the nature of the other parent, with regard to each trait, seemed to have disappeared. Mendel discovered that that traits that were expressed were the dominant traits and the traits which “disappeared” were recessive.

Dominant and Recessive Traits In Mendel’s experiments, the allele for tall plants was dominant and the allele for short plants was recessive. Likewise, the allele for yellow seeds was dominant over the recessive allele for green seeds

Segregation Mendel wanted to find out what had happened to the recessive alleles. To find out, Mendel allowed all seven kinds of F1 hybrids to self-pollinate. The offspring of an F1 cross are called the F2 generation. The F2 offspring of Mendel’s experiment are shown.

The F1 Cross Mendel wanted to find out what had happened to the recessive alleles. To find out, Mendel allowed all seven kinds of F1 hybrids to self-pollinate. The offspring of an F1 cross are called the F2 generation.

The Formation of Gametes Let’s assume that each F1 plant—all of which were tall—inherited an allele for tallness from its tall parent and an allele for shortness from its short parent.

The Formation of Gametes When each parent, or F1 adult, produces gametes, the alleles for each gene segregate from one another, so that each gamete carries only one allele for each gene.

The Formation of Gametes A capital letter represents a dominant allele. A lowercase letter represents a recessive allele. Each F1 plant in Mendel’s cross produced two kinds of gametes—those with the allele for tallness (T) and those with the allele for shortness (t).

Explaining the F1 Cross How did this separation, or segregation, of alleles occur? Mendel suggested that the alleles for tallness and shortness in the F1 plants must have segregated from each other during the formation of the sex cells, or gametes.

Mendel’s Basic Observations Something is passed on from parent to offspring This Something controls different traits This Something remains unchanged during the life of an organism This Something is passed on in the gametes (reproductive or sex cells – sperm & egg)

Mendel’s Gene Hypothesis (based on his observations) A hereditary unit of information, or a gene, is passed from parent to offspring, for example, the gene for the characteristic “eye color” An allele is one of the possible versions of the gene, for example, the allele for the brown eyes or the allele for the blue eyes

The Formation of Gametes Whenever each of two gametes carried the t allele and then paired with the other gamete to produce an F2 plant, that plant was short. Every time one or more gametes carried the T allele and paired together, they produced a tall plant. The F2 generation had new combinations of alleles.

Mendel’s Gene Hypotheses The presence of different alleles is responsible for the variation in the appearance of an organism An organism always has 2 genes for each characteristic, one inherited from each parent (one from Mom, one from Dad) If the alleles on both of the genes are the same, then the organism is purebred

Mendel’s Gene Hypotheses If the 2 alleles are different, then the organism is a hybrid for the characteristic. One trait will be dominant (brown eyes) and one (blue eyes) will be recessive. *Only the dominant trait will be expressed. When both genes exist, the dominant trait hides the recessive trait*

Law of Independent Assortment Most traits are independently sorted Mendel found that of the 7 characteristics he studied in the peas, none of them had anything to do with the others For example, the shape of a pea pod has NO influence on whether the plant is tall or short This is the law of independent assortment

Punnet Squares, Dominance & Recessiveness One gene is indicated by a single letter. Capital = Dominant; lower Case = recessive Organisms with matching alleles are homozygous for that trait, so all purebreds are homozygous Homozygous dominant = purebred for dominant trait = represented as PP Homozygous recessive = purebred for recessive trait = represented as pp

Phenotype and Genotype Phenotype: the outward appearance of a characteristic in an organism eg: color = purple flower color phenotype or white flower color phenotype Genotype: the genetic makeup of an organism A plant may have the genes for both a purple and a white flower, although it looks purple

Punnet Square Example Angus Cattle - Black coat color is dominant BB = Homozygous Dominant and Black Bb = Heterozygous and is black bb = Homozygous recessive and red A heterozygous bull is mated to 50 homozygous recessive cows. A Rancher wants to know: How many calves will be black? How many calves will be red? What is the genotypic & phenotypic ratios of the calves?

Punnet Square

What are the results for the Offspring? How many of each type of cow?

Results 2 heterozygous = Bb 2 Homozygous Recessive bb 25 Black, 25 Red Genotypic ratio = 0:2:2 Phenotypic ratio = 2 Black: 2 Red Results

What is a Mutation? Sometimes, stuff happens….. It’s a change in genotype, dramatically different from what is expected For example, red hair from brunette parents, or the loss or extra body parts Lethal Mutation: causes death at birth Beneficial Mutation: loss of tail in lambs

Simple Dominant/Recessive Traits Many human genes are inherited as dominant or recessive traits just like the traits Mendel studied in peas Ear lobes provide an example of this: Unattached ear lobes are inherited as a dominant trait. Attached ear lobes are inherited as a recessive trait.

Simple Dominant/Recessive Traits Having a bent little finger is a dominant trait

Dominant/Recessive Human Traits Albinism - Characterized by an absence of pigment from the skin, eyes and hair. Albino individuals typically have very pale white skin, light blue or pink eyes, and light blond or white hair. Albinism is a recessive trait. Brown teeth - Teeth have a brown color that is not the result of poor oral hygiene or antibiotics taken as a child. White teeth are dominant.

Dominant/Recessive Human Traits Cleft chin - A noticeable indentation at the center of the chin. For examples think of Kirk Douglas and Michael Jackson (after plastic surgery). Having a cleft chin is dominant to a smooth chin. Double-jointed thumbs - This is commonly called a hitchhiker's thumb. The thumb can bend back at almost 90 degrees. Hitchhiker's thumb is a recessive trait, but it may vary in its expression.

Dominant/Recessive Human Traits Hand folding - When the hands are folded either the left or right thumb will be on top. Left thumb on top is dominant. Mid-digital hair - Hair growing from the middle section of each finger. Hair presence is dominant. PTC tasting - Phenylthiocarbamide (PTC) is a bitter tasting chemical that cannot be tasted by some individuals. PTC tasters taste PTC as bitter, non-tasters taste nothing when given PTC. Tasting is dominant to the inability to taste PTC.

Dominant/Recessive Human Traits Tongue rolling - Tongue rolling is the ability to form a tube with your tongue. Rolling is dominant. Widow's peak - A sharp point in the hairline that points toward the nose. Having a widow's peak is dominant to a smooth hairline.

Sex Influenced Human Traits Baldness - Loss of hair from the scalp following puberty. A sex influenced trait that is most commonly fully expressed (as a dominant trait) in males, but carried on an autosome Index finger shorter than ring finger - The index finger (next to your thumb) is longer than the ring finger (next to your little finger). Check the class data to see if the frequency is different for the different sexes.

Sex Linked Human Traits Color blindness - Inability to distinguish between colors of the same intensity. There may be two types, red green color blindness in which individuals perceive red and green as the same color, and complete color blindness in which all colors are perceived as being the same and vision is the functional equivalent of black and white television. Both types of colorblindness are sex linked traits.