Download Heredity

Unit 5: Heredity
Heredity – the passing of physical characteristics from parent to offspring
Trait – a characteristic that a parent can pass on to its offspring through its genes
Genetics – the scientific study of heredity
Gregor Mendel – a priest who, from 1856 to 1863, studied the characteristics of 28 000 pea plants
and founded the science of genetics
Mendel started with two purebred plants with different characteristics
Purebred organisms are the offspring of many generations that retain the same trait
Mendel cross bred purebred short plants with purebred tall plants
Cross breeding is the fertilization of a purebred organism with one trait with a purebred
organism with a different trait – a purebred tall plant with a purebred short plant, for
example
Today, scientists call the parent plants the P generation (P = parental)
The first generation, or filial plants, are called the F1 generation
The second generation plants are called the F2 generation
Results of Mendel’s experiment with tall and short pea plants
P:
tall
X
short
F1:
all offspring tall
F2:
¾ offspring tall and ¼ offspring short
Dominant allele – a trait that is always present when the allele is present
Genes – factors that control a trait
Alleles – the different forms of a gene
Recessive allele – a trait that is hidden whenever the dominant allele is present
Which allele is dominant according to Mendel’s data?
Answer: the tall allele is dominant
Which allele is recessive according to Mendel’s data?
Answer: the short allele is recessive
The F1 plants are called hybrids
Hybrid – an organism that has two different alleles for a trait
Probability – a number that describes how likely it is that an event will occur
Mathematics: probability and fractions
Flip a coin: how likely is it that the coin will land on heads?
There is an equal chance that it will land on heads vs. tails
Since there are two sides to the coin, the chance of heads is 1 out of 2
Scientists record this probability as ½
Flip the coin a second time: how likely is it that the coin will land on heads?
There is still an equal chance that it will land on heads on the second toss
The first toss has no effect on the second toss
Independence of events – when the result of one event in a probability has no effect on the next
event, scientists call this independence of events
Mendel realized that the ratios of types of offspring in his pea plant experiments represented a
mathematical probability
Remember, he reported the number of tall plants as ¾ and the number of short plants as ¼
Punnett Squares – a chart that shows all the possible combinations of alleles that can result from a
genetic cross
Think of a Punnett Square as a tool that applies the laws of probability to genetics
Symbols for alleles
Geneticists use letters to represent alleles
Dominant alleles are represented by capital letters
Recessive alleles of the same gene are represented by a lower case version of the same letter used
for the dominant allele
Alleles come in pairs because chromosomes come in pairs
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Genotype – an organism’s genetic makeup or allele combinations
Homozygous – having two identical alleles for a trait
Heterozygous – having two different alleles for a trait
Phenotype – an organism’s physical appearance or visible traits
Examples:
Genotype
Phenotype
Purebred tall pea plant
TT (homozygous)
tall stem
Purebred short pea plant
tt (homozygous)
short stem
Hybrid pea plant
Tt (heterozygous)
tall stem
Genetic expression
Dominance – only the dominant allele is expressed in a
hybrid
Note that both hybrid Rr to the right are red
Recessive – expressed only in a homozygous organism
with two recessive alleles
Note that only the homozygous rr is white
Codominance – heterozygous alleles are
both expressed with no blending
Incomplete dominance – heterozygous
alleles are blended
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