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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
1. For each inherited trait, an individual has a copy of that gene from each parent.
2. There are alternate versions of genes, called alleles. (ex: freckles or no freckles)
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Hypothesis continued
3. 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
1. Law of Segregation- two alleles separate when gametes are formed
2. Law of Independent Assortment- alleles of different genes separate independently of one another during gamete formation
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fertilization produces heterozygous offspring
meiosis II
meiosis I
(chromosomes duplicated
before meiosis)
Homozygous dominant parent
Homozygous recessive parent
(gametes) (gametes)
Fig. 11-5, p.172
Mendel’s Law of
Segregation
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Nucleus of a diploid (2n)reproductive cell with two pairs of homologouschromosomes
Possible alignmentsof the two homologouschromosomes duringmetaphase I of meiosis
The resulting alignments at metaphase II
Allelic combinationspossible in gametes
1/4 AB 1/4 ab 1/4 Ab 1/4 aB
Fig. 11-8, p.174
Independent Assortment
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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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Fig. 11-10a, p.176
Range of genotypes:
IAIA IBIB
IAi IAIB IBi ii
BloodTypes:
or
A AB B O
or
ABO Blood Type
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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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Formation of sperm and egg
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