Essential Question: How can we predict the phenotype that offspring will inherit from their parents? Science 7

Next Test Date: Science “Human Reproduction and Heredity” Unit Test WEDNESDAY, MARCH 23

Homework: Complete Punnett square worksheet for Monday. You MUST SHOW ALL WORK for credit!

Traits: What are inherited traits? Inherited traits are traits that are determined by your DNA; they come from your parents What are acquired traits? Acquired traits are things that you learn or pick up throughout your life; they are NOT determined by your DNA ___________________ is the study of how inherited traits are passed from parents to offspring. Heredity A ___________________ IS a specific section of your DNA that determines ONE SPECIFIC TRAIT that you possess. Gene

What are “genes”? A GENE is a specific section of your DNA that determines one particular trait, or has a single effect on many traits. It is the GENES of an individual organism that make it unique compared to the other members of its species Only a very small percentage of your DNA is made up of genes– most of your DNA is shared with all other members of your species

What are “genes”? Each individual GENE is split between a pair of chromosomes– half of the gene on one of the chromosomes in the pair, and half on the other side These “half-sections” found on each chromosome in a pair are called ALLELES. Every gene in your DNA is composed of two alleles, one allele on each chromosome in the pair

What is a gene? When we talk about genes while studying heredity, we can represent each gene with a pair of letters. Each letter in the pair represents one of the alleles for the gene. The letters will be some combination of two capitals, two lower-case, or one capital and one lower- case. Always use the same letter for each allele, just alter the capitalization (with one exception) What letters we use don’t matter– they are just variables. The same way that “x+1=2” and “y+1=2” mean the same thing, just with a different variable. This two letter combination is referred to as the GENOTYPE for a particular gene– the genotype shows what ALLELES you have for a gene

Genotype vs. Phenotype GENOTYPE: Genotype is the alleles that you have in your DNA for a particular gene Genotype is a set of two letter that are used to represent a particular set of alleles. For example, we use “XX” as the genotype for females. Each “X” represents one allele. We also use “XY” to represent the genotype for males. The “X” represents one allele. The “Y” represents the second allele.

Genotype vs. Phenotype PHENOTYPE: Phenotype is the trait that you actually SHOW for a particular gene. DIFFERENT GENOTYPES CAN PRODUCE THE SAME PHENOTYPE! For example, we use “XX” as the genotype for females. But the phenotype would be “FEMALE” We also use “XY” to represent the genotype for males. But the phenotype would be “MALE”

Genotype vs. Phenotype JUST BECAUSE TWO PEOPLE SHOW THE SAME TRAIT (PHENOTYPE), THEY DON’T NECESSARILY HAVE THE SAME GENOTYPE!

Dominant Alleles vs. Recessive Alleles DOMINANT ALLELES: Dominant alleles will ALWAYS be the phenotype, if they are present in the genotype for a gene. We represent dominant alleles with a CAPITAL letter. (For example: “A”) RECESSIVE ALLELES: Recessive alleles will NOT be the phenotype, unless both alleles are recessive. We represent recessive alleles with lower-case letters. (For example: “a”)

Today’s Topic: How can we predict the phenotype of the offspring of two parents?

Examples:

Heterozygous Genotypes vs. Homozygous Genotypes Heterozygous genotype (hybrid): A genotype with ONE DOMINANT allele (“A”) and ONE RECESSIVE allele (“a”). This will always show the DOMINANT TRAIT as your phenotype Homozygous genotype (pure): A genotype with either: TWO DOMINANT ALLELES (which will show the DOMINANT TRAIT as your phenotype Or TWO RECESSIVE ALLELES (which will show the RECESSIVE TRAIT as your phenotype

Punnett Squares: Punnett squares are used to determine the probability of certain traits appearing in the children of two parents One parent’s genotype is written across the top; the other parent’s genotype is written across the side You then “cross” the letters to create the four possible genotypes that the child could have Using these numbers, you can then determine the probability of certain phenotypes appearing

Sample Problem #1: Punnett Squares A man and a woman have a child. Using their genotypes for sex, fill in the Punnett square on the right. a) What percentage of offspring are likely to be female? What percentage of offspring are likely to be male? b) Explain… which chromosome is DOMINANT for the sex of a child? How do you know? c) Is it possible to have a “YY” genotype for a child’s sex chromosomes? How do you know?

Sample Problem #2: Punnett Squares (The following is hypothetical. Do not assume that these genes actually exist). Let's say that in seals, the gene for the length of the whiskers has two alleles. The dominant allele (W) codes long whiskers & the recessive allele (w) codes for short whiskers. What percentage of offspring would be expected to have short whiskers from the cross of two long-whiskered seals, one that is homozygous dominant and one that is heterozygous?