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Genetics Chapter 11
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Heredity Passing of traits from parent to offspring
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Genetics The study of heredity
Gregor Mendel- Austrian monk known as the “father of genetics”
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Mendel’s Four Hypotheses
For each inherited trait, an individual has a copy of that gene from each parent. There are alternate versions of genes, called alleles. (ex: freckles or no freckles)
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Hypothesis continued When 2 different alleles occur together, one of them may be completely expressed (dominant), while the other may not be observed (recessive). 4. Alleles separate independently, so that each gamete only carries 1 allele for that trait.
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Monohybrid cross Mendel began with 1 trait (monohybrid) crosses
He bred pea plants to produce several generations P- the parent generation F1 - the first filial generation F2 - second filial generation
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Mendel concluded that each parent has two separate “factors” for a particular trait
“Factors” are now called genes.
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Mendel’s Laws of Heredity
Law of Segregation- two alleles separate when gametes are formed Law of Independent Assortment- alleles of different genes separate independently of one another during gamete formation
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Mendel’s Law of Segregation
Homozygous dominant parent Homozygous recessive parent Mendel’s Law of Segregation (chromosomes duplicated before meiosis) meiosis I meiosis II (gametes) (gametes) fertilization produces heterozygous offspring Fig. 11-5, p.172
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Independent Assortment
Nucleus of a diploid (2n) reproductive cell with two pairs of homologous chromosomes Possible alignments of the two homologous chromosomes during metaphase I of meiosis The resulting alignments at metaphase II Allelic combinations possible in gametes 1/4 AB 1/4 ab 1/4 Ab 1/4 aB Fig. 11-8, p.174
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Some modern genetic terms
Alleles are represented by letters Dominant allele is a capital letter Recessive allele is a lower case letter
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Homozygous- identical alleles for a specific trait (BB, FF, rr, tt)
Heterozygous- alleles are different for a specific trait (Bb, Ff, Rr, Tt) (aka “hybrid”) Genotype- the set of alleles an individual inherits for a trait (i.e.- Rr, Ww, ff) Phenotype- the physical expression of a trait (i.e. blue eyes, freckles, dimples)
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Punnett Square Predicts possible outcomes of traits
Shows all possible outcomes of a genetic cross
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Probability (likelihood that an event will occur) of a genetic outcome can be predicted
Ex: cross two heterozygous individuals Aa x Aa predict ratios of 3:1 for phenotype and 1:2:1 for genotype
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Practice Curly hair is dominant over straight hair. A man with straight hair and a woman who is heterozygous for curly hair have a child. What is the probability that this child will have straight hair?
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Incomplete Dominance- when an individual shows a combination of the inherited alleles.
Ex: red snapdragon x white snapdragon will produce a pink snapdragon straight hair x curly hair = wavy hair
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Codominance- 2 dominant alleles are expressed at the same time
Ex: Roan horses show both red and white hairs in equal numbers
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Patterns of heredity can be very complex
Sex-linked trait- a trait whose alleles is located on the X chromosome (most are recessive) Polygenic trait- trait where several genes influence the outcome (eye color, hair color, skin color, height, weight)
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Multiple Alleles- genes with 3 or more alleles
- Blood type is an example of multiple alleles because we have A, B and O alleles and 4 possible blood type outcomes (A, B, AB and O)
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ABO Blood Type Range of genotypes: IAIA IBIB or or IAi IAIB IBi ii
Fig a, p.176
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Diploid Number of chromosomes found in the body or somatic cells of an organism “2n”
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Haploid Number of chromosomes found in the gametes of an organism “n”
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Meiosis The type of cell division that produces gametes
Gametes = sex cells female gamete = egg or ovum (plural = ova) male gamete = sperm
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Meiosis
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Formation of sperm and egg
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