Name: _________________________________
Core: _______
INTRO TO HEREDITY – November 30, 2015
Heredity: the passing of traits from parents to offspring.
Trait: A physical or behavioral characteristic that describes an organism (Ex: height; eye color; endurance)
Genetics = study of genes, heredity, and genetic variation.
Importance of Gregor Mendel
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Austrian botanist monk
Mid 1800s
Formed the basic laws of heredity
His work laid the foundation to the study of heredity
Mendel is referred to as “The Father of Genetics.”
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Mendel studied garden pea plants.
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Observed differences in multiple traits over many generations, because pea plants reproduce rapidly, and have
many visible traits such as: plant height, seed color, pod color, seed shape, pod shape
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Mendel noticed that some plants always produced offspring that had a form of a trait exactly like the parent
plant (he called these “purebred”).
Example: Purebred short plants always produced short offspring and purebred tall plants always produced tall
offspring.
Mendel’s First Experiment:
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Mendel crossed purebred plants with opposite forms of a trait.
Example: Purebred tall plants were crossed with purebred short plants.
Mendel observed that all of the offspring grew to be tall plants. None resembled the short parent.
P stands for “parent” and F# stands for “filial” (son).
Mendel’s Second Experiment:
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Mendel then crossed two of the offspring tall plants produced from his first experiment.
Offspring ratio in F2 generation = ¾ tall and ¼ short
To his surprise, Mendel observed that this generation had a mix of tall and short plants, even though none of the
F1 parents were short!
Mendel’s first law, the Law of Segregation, has three parts.
From his experiments, Mendel concluded that:
1. Traits are handed down through “hereditary factors” in the sperm and egg.
2. Because offspring obtain hereditary factors from both parents, each organism must contain two factors for
every trait.
3. The factors in a pair segregate (separate) during the formation of sex cells, and each sperm or egg receives
only one member of the pair.
Dominant and Recessive Alleles
One factor (gene) in a pair may mask, or hide, the other factor.
Example: When he crossed a purebred tall plant with a purebred short plant, all offspring were tall even though
all the F1 offspring should have both tall and short factors. (The tallness factor masked the shortness factor.)
Genes: a section of DNA that determines a trait.
Alleles: the different forms of a gene.
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Alleles are abbreviated with a letter.
Example:
T = allele for “tall” height
t = allele for “short” height
Every gene has two alleles (1 from mom & 1 from dad).
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Alternative versions of genes (different alleles) result in variations in inherited traits.
Example: The gene for flower color in pea plants exists in two versions, one for purple flowers and the
other for white flowers
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Each gene resides at a specific location on a specific chromosome (remember that there are multiple
chromosomes)
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If the two alleles for a gene are different, the dominant allele will determine the organism’s appearance and
“mask” the other allele.
- Dominant alleles are shown with a Capital letter.
- Dominant alleles are “STRONGER.”
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The recessive allele will only determine the organism’s appearance if both alleles on the gene are recessive.
- Recessive alleles are shown with a lower-case letter.
- Recessive alleles are “weaker.”
Mendel observed a variety of dominant alleles in pea plants other than the tall allele. For instance, a plant that is
heterozygous (Yy) for seed color will always have yellow seeds.
However, a plant that is a heterozygous for pod color (Gg) always displays the green allele.
In addition, round seeds are dominant over wrinkled seeds, and smooth pods are dominant over wrinkled pods.
Homozygous Genes
We know now that Mendel’s “purebred” plants had two identical alleles for a particular gene. The modern scientific
term for “purebred” is homozygous.
Homozygous: having the same allele for a particular gene (NN or nn).
Example:
A homozygous tall plant has two tall alleles (TT) and will only
contribute a tall allele (T).
A homozygous short plant has two short alleles (tt) and will only
contribute a short allele (t).
Heterozygous Genes
When the two types of homozygous plants were crossed the offspring received one tall gene and one short gene from
the parent plants.
All offspring contained both alleles (1 tall allele and 1 short allele).
Heterozygous: possessing two different forms of a gene, one inherited by each parent (Nn).
Although the offspring have both a tall and a short allele, only the tall allele is expressed (the tall allele is
dominant and “masks” the short allele).
Genotype vs. Phenotype
Genotype: the two alleles that an organism has for a trait.
(Example: AA, Aa, aa)
Phenotype: the physical trait that the organism shows or “expresses”.
(Example: Does the butterfly look dark or light in color?)
Law of Independent Assortment: Each pair of genes separate independently of each other in the production of sex cells
(egg or sperm).
Example: Each sex cell will have only 1 allele for every gene (haploid). The plant will donate either a yellow or
green seed allele, or a wrinkled or a smooth pod shape. If the plant donates the yellow seed allele it does not
mean that it will also donate the wrinkled pod allele.
The donation of one allele from each pair is independent of any other pair.
Remember:
1) There are 2 alleles for every gene.
2) Dominant alleles are “stronger” and Capitalized.
3) Recessive alleles are “weaker” and lower-case.
4) Recessive traits will only be shown if both alleles are recessive.
What are the possible allele combinations for these 2 plants?
Dominant trait: ____________________
Allele: ____
Recessive trait: ____________________
Allele: ____
Possible genotype combinations: _________________________
Possible phenotype combinations: ________________________