Gregor Mendel Gregor Mendel was an Austrian monk who studied genetics and observed how these traits were inherited over several generations. Great slide!

Gregor Mendel His work was REALLY important to understanding inheritance. He studied what happened when he bred different kinds of pea plants (ex. green vs. yellow, short vs. tall, round vs. wrinkly) From his experiments he came up with several conclusions. Great slide!

What did he learn from his studies? Biological inheritance is determined by “factors” (genes within our DNA )that are passed on from one generation to the next. Different forms of these genes determine our physical traits. For every gene, we inherit two alleles (one allele from each parent). Great slide!

What did he learn from his studies? trait: specific characteristic that varies from one individual to another allele: one of a number of different forms of a gene locus: the location of a gene on a chromosome Great slide!

Mendel’s three Principles Principle (Law) of Segregation states that the two different alleles for a gene segregate independenty from one another during meiosis Great slide!

Mendel’s three Principles Principle (Law) of Independent Assortment states that different traits sort and are inherited independently of one another when sex cells are formed Great slide!

Mendel’s Principles (A Summary) For every gene, we inherit two copies (one allele from each parent) Blonde Hair Brown Eyes Great slide! Blonde Hair Blue Eyes Brown Hair Brown Eyes

If we have two sets of manuals, how do we know which directions to follow? If we have two alleles for every trait, how does our body pick which one to express? Blonde Hair Brown Eyes Great slide! Blonde Hair Blue Eyes Brown Hair Brown Eyes

if we have two sets of manuals, how do we know which directions to follow? When we get directions from each parent, it is a combination of the different directions that determine our traits. Great slide!

Mendel’s three Principles Principle (Law) of Dominance states that when two or more forms of a gene (alleles) exist for a single trait, some alleles are dominant while others are recessive Great slide!

Principle of Dominance Some alleles are dominant and others are recessive . Organisms with a dominant allele for a particular trait WILL exhibit that trait, because the dominant allele overpowers the recessive allele. Great slide!

Principle of Dominance A recessive trait will only show in in the organism when BOTH alleles are recessive – that is, when the dominant allele is NOT present. Great slide!

PUNNETT SQUARES A Punnett square is a useful tool used to determine or predict the probability that offspring will have certain traits after a genetic cross. Great slide!

PUNNETT SQUARES: probability Probability: the likelihood that a particular event will occur Because each organism has two alleles for each gene there is an equal (50/50) chance of giving either allele to an offspring Great slide!

PUNNETT SQUARES: terms Genotype: genetic makeup of an organism (alleles) Genotype sounds like genes Phenotype: physical characteristics/appearance of an organism Phenotype sounds like physical Great slide!

PUNNETT SQUARES: terms Homozygous: an organism with two identical alleles for a particular trait Heterozygous: an organism with two different alleles for the same trait Great slide!

Let’s use those words in an Example Homozygous for the tall trait – “TT” is the genotype, “tall” is the phenotype Heterozygous for the tall trait – “Tt” is the genotype, “tall” is the phenotype Homozygous for the short trait – “tt” is the genotype, “short” is the phenotype Great slide!

PUNNETT SQUARES: Rules Use letters to stand for alleles Dominant alleles are noted with a CAPITAL ( D ) letter Recessive alleles for the same trait are noted with the lowercase ( d ) of the same letter. Great slide!

PUNNETT SQUARES: Rules Parental genotypes are shown along the top and side of the Punnett square. Write one parent’s alleles on the top, and write the other on the side Put one letter next to each cell, and drag alleles across or down so that there are two alleles in each square. Great slide!

PUNNETT SQUARES: Rules Each of the four squares represents a possible genotype of the offspring These are just probabilities! Great slide!

Punnett square practice 1. A homozygous tall (TT) pea plant is crossed with a homozygous short (tt) pea plant: Great slide!

Punnett square practice A homozygous red (RR) flower is crossed with a heterozygous red (Rr) flower: Great slide!

Punnett square practice A heterozygous funny (Ff) bird is crossed with a heterozygous funny (Ff) bird: Great slide!

Offspring Ratios When you have two homozygous parents? Genotype Ratio: ________ Phenotype Ratio:________ When you have one homozygous and one heterozygous parent? When you have two heterozygous parents? Great slide!

Multiple generations Sometimes, we need to do a Punnett square to follow more than one generation. Generations: P  F1  F2 parents first filial second filial generation generation Great slide!

Test crosses If you need to figure out the genotype of a parent with the dominant trait, you can do a test cross. This is done by crossing that parent with a homozygous recessive organism. Great slide!

Test crosses If any offspring show the recessive allele, you know that the dominant parent is heterozygous. Great slide!

Test crosses If none of the offspring show the recessive allele, you know that the dominant parent is homozygous. Great slide!

Example: A man with free earlobes wants to know if he is homozygous or heterozygous for that trait. He marries a woman with attached earlobes (the recessive trait). His first child has attached earlobes. The man has thus found out that he is _________________________.

Going Beyond Dominant and Recessive Alleles Some alleles are neither dominant nor recessive – many traits are controlled by more than one allele or by multiple genes… Great slide!

Incomplete dominance Incomplete dominance: a situation in which one allele is not completely dominant over another Because neither trait dominates the other, sometimes the alleles are written differently (ex. RW, CrCw, DD’) Examples are: flowers (red/pink/white), hair (curly/wavy/straight), fur (long/short) Great slide!

Identifying Incomplete dominance Notice that the offspring show a third phenotype that is different from the parents The offspring phenotype is a MIX or BLEND of the parental traits Great slide!

Identifying Incomplete dominance What happens when we cross a red (RR) flower with a white (WW) flower? The offspring look pink Great slide!

codominance codominance: a situation in which both alleles of a gene contribute to the phenotype of the organism What does the prefix “co-“ mean? to share Because neither trait dominates the other, sometimes the alleles are written differently (ex. RW, CrCw, DD’) Examples are: cows (red/roan/white), flowers (red/spotted/white), blood type (A/AB/B) Great slide!

Identifying codominance Notice that the offspring show a third phenotype that is different from the parents The offspring phenotype has both parental traits appearing TOGETHER Great slide!

Identifying codominance What happens when we cross a pure red (RR) cow with a pure white (WW) cow? The offspring look roan - spotted with red & white Great slide!

Identifying codominance What happens when we cross a pure red (RR) cow with a pure white (WW) cow? The offspring look roan - spotted with red & white Great slide!

Polygenic traits Traits controlled by two or more genes are said to be polygenic. Polygenic traits show a wide range of phenotypes because different combinations of alleles on these genes produce many different traits. Great slide!

Polygenic traits For example, hair color and eye color are controlled by more than one gene and therefore have more than two phenotypes. Great slide!

Polygenic traits And, the wide range of skin color in humans is due to more than four different genes that control this trait. Great slide!