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Genetics: The Science of Heredity
What is Heredity?
What did Mendel observe?
Gregor Mendel: Known as the
Father of Genetics, he unlocked the
key to understanding heredity using
pea plants. He was able to obtain
predictable, repeatable results using
plants that could both self-pollinate & cross-pollinate.
Heredity the passing of traits/physical characteristics from parents to
offspring
Genetics the scientific study of heredity
Fertilization the joining of egg and sperm
cell (the beginning of the formation of new organism)
Genes segments of DNA that carry hereditary instructions on
chromosomes & passed from parent to offspring
(factors that control a trait)
Alleles alternative forms of a gene that govern the same characteristics (ex. Eye color)
How do alleles affect inheritance?
An organism's traits are controlled by the alleles it inherits from its parents. Some alleles
are dominant, while other alleles are recessive.
Dominant trait trait observed when at least one dominant allele for a characteristic is
inherited
Recessive trait trait that is apparent only when 2 recessive alleles for the characteristic are
inherited
Homozygous/Purebred having 2 copies of the same allele for a trait (ex. HH or hh)
Heterozygous/Hybrid having 2 different alleles for a trait (ex. Rr or RW)
Probability & Heredity
How is probability related to inheritance?
In a genetic cross, the combination of alleles that parents can pass to an offspring is based on
probability = the mathematical chance that an event will occur
Punnett Square (designed by Reginald C. Punnett) a chart that shows all the possible ways alleles
can combine in a genetic cross
Parent generation (Purebred purple flowers - PP x Purebred white flowers - pp)
P
p
p
P
Pp
Pp
Pp
Pp
= 100% purple
F2 generation (self pollination of the offspring from the F1 generation: Pp x Pp)
P
p
P
PP
Pp
p
Pp
pp
= 75% purple
= 25% white
(recessive trait reappears)
What are phenotype & genotype?
Genotype the inherited combination of alleles; genetic make up
Phenotype an organism’s inherited appearance; visible appearance
Patterns of Inheritance
How are most traits inherited?
Most traits are the result of complex patterns of inheritance.
Codominance both alleles for a gene
are expressed equally (and individually)
Incomplete dominance one allele is
only partially dominant (blending occurs)
Multiple Alleles three or more possible alleles determine the trait (an organism can still
only inherit 2 alleles for a gene, one from each parent, but there are more possible
genotypes with multiple alleles than with just 2 alleles; example = blood type)
Polygenic Inheritance more than one gene affects a trait (broad range of genotypes:
human height, skin color, and eye color)
Inherited Trait traits organisms are born with (height, freckles, eye color)
Acquired Trait traits organisms acquire (language a person speaks, haircuts, piercings,
tattoos)
How do genes and the environment interact?
Environmental factors can influence the way genes are expressed.
Examples:
• slider turtles & temperature
• inherited ability and opportunity to learn
• tobacco smoke and gene triggers
Changes in body cells cannot be passed on to offspring. Only changes in sex cells –
eggs & sperm can be passed to offspring.
Chromosomes & Inheritance
How are chromosomes, genes, and inheritance related?
According to the chromosome theory of inheritance, genes pass from
parents to their offspring on chromosomes.
Chromosome: coiled structure of DNA & protein that forms in the
nucleus during cell division; passed from one generation to the next
What happens during meiosis?
Meiosis produces new cells with ½ the usual number of
chromosomes. This process creates sex cells. The chromosomes are
copied/duplicated only once & nucleus divides twice.