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Mills Biology
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California State Standards 2.c Students know how random chromosome segregation explains the probability that a particular allele will be in a gamete 2.d Students know new combinations of alleles may be generated in a zygote through the fusion of male and female gametes 3.a Students know how to predict the probable outcome of phenotypes in a genetic cross from the genotypes of the parents and mode of inheritance. 3.b Students know the genetic basis for Mendel’s laws of segregation and independent assortment.
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5 Essential Questions: Who do we inherit our genetic traits from, and how is this information passed to us? What are dominant and recessive traits? Examples of each? What is an allele? What are the differences between genotypes and phenotypes? Why do our genetic traits depend on meiosis?
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We already know… Sex cells, or gametes (sperm and egg) are created in meiosis. The result of meiosis is haploid cells with HALF the normal # of chromosomes (23). When the haploid sperm cell and haploid egg cell meet, they will make a diploid cell with 46 chromosomes. This is the beginning of YOU.
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Sex cells are really important… Because their genetic information controls what traits kids will inherit from their parents. Chromosomes contain genes, which control traits. We get 23 chromosomes from mom, and 23 from dad.
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Some inherited traits are….
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Traits are controlled by genes The genes for traits are located on the pairs of chromosomes we get from mom and dad. Some of these genes control the same trait (eye color) but may be different versions of that trait (blue, brown, green eyes) Different versions of the same trait are called alleles.
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Locus for eye color Allele for brown eyes Allele for blue eyes
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Genetic Traits Traits can either be dominant or recessive (brown eyes are dominant, blue eyes are recessive) We write the alleles for traits as letters. Dominant traits are written as a capital letter and recessive traits are written as lowercase letters. B=brown eyes and b= not brown eyes.
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DOMINANT TRAITS Will be the physical trait no matter what other allele they are paired with. They are written as a capital letter. What are some dominant traits?? RECESSIVE TRAITS Are the physical trait only when paired with another recessive allele. They are written as a lowercase letter What are some recessive traits??
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Allele Pairs HOMOZYGOUS Two of the same alleles for a trait (BB, bb) HETEROZYGOUS Two different alleles for a trait (Bb)
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Expression of traits Genotype An allele pair (BB, Bb, bb) We can’t always tell the genotype from the physical trait. Phenotype The outward expression of the allele Hair color, height, left/right handed, nose shape, # of arms
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How do we know this? Gregor Mendel (1850’s) the “father of genetics” Began experimenting with pea plants. He wanted to see how traits were passed from generation to generation of plants. He wanted to see what would happen if he crossed two plants with separate traits (like color – yellow and green) He determined that there must be two forms of each trait in the plants, that is controlled by a factor. We call that “factor” an ALLELE.
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Genetics using Punnett Squares
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Last but not least… There are two genetic laws that we need to know: The Law of Segregation: Two alleles for each trait separate during meiosis. During fertilization, two alleles for that trait unite. When alleles from the parents unite to form a heterozygous allele pair, this is called a hybrid.
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The Law of Independent Assortment: Random distribution of alleles occurs during gamete formation (meiosis), because chromosomes sort independently. In other words, alleles mix up randomly when chromosomes split in meiosis.
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What is a Punnett Square? A tool that helps us predict the possible children that could come from two parents. It shows the allelic combinations of offspring
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How do we use a Punnett Square? Make a 4-box grid BbBb B b Put one parent’s genotype on the top. Then put the other parent’s genotype on the left.
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How do we use a Punnett Square? After labeling the genotypes of the parents, fill in the rest of the grid. Write capital letters FIRST B b BbBb BB Bb bb
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Probability After we fill out the Punnett Square, we can figure out the probability of offspring. Example: 1:2:1 Example: 3:1 Example: 4:0 Because there are four squares, every square is 25% of the probability.
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Monohybrid Cross: A Punnett Square that only focuses on 1 trait. We have only looked at monohybrid crosses. Dihybrid Cross: A Punnett Square that looks at 2 traits at a time.
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Complex Inheritance and Human Heredity
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Recessive Genetic Disorders A person will only have these disorders if they are homozygous for the disorder (pg. 297) Cystic Fibrosis Albinism Tay-Sachs disease Sickle Cell Anemia Galactosemia A carrier is a person who is heterozygous for the trait (they carry it but do not have the disorder)
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Dominant Genetic Disorders People who are heterozygous for these disorders will have them. Huntington’s Disease Dwarfism (Achondroplasia) Marfan Syndrome
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Incomplete Dominance When a heterozygous phenotype is in the middle of two homozygous phenotypes. Example: A homozygous red plant (RR) crossed with a homozygous white plant (rr) will be pink (Rr)
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Co-dominance In Co-dominance, both alleles in a heterozygous allele pair are expressed in the phenotype, instead of just the dominant one.
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Sex – Linked Traits Males have one X chromosome, and females have two. Sex-linked traits are located on the x chromosome Red-green color blindness (page 307) Hemophilia
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Nondisjunction When chromosomes don’t separate correctly in meiosis. Page 312 In the child, this can result in an extra chromosome, or not enough chromosomes. Example: Down Syndrome
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