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STUDY GUIDE – GENETICS
Genetics- Study of how traits are inherited or passed between generations.
Chromosomes- in the nucleus, controls what the cell does & controls all your traits.
Humans = 46
Karyotype – a map of an organisms chromosomes
Human Genome Project – a map of all the genes on the human chromosomes
Genes- Small sections of chromosome that are responsible for making 1 trait.
Pairs = 1 from mom, 1 from dad
Alleles – the different forms of a gene (2)
Dominant Gene- stronger gene of a gene pair. Capital letter (A)
Recessive Gene- weaker gene of a gene pair. Lower case letter (a).
If a person has both a dominant & recessive gene, the recessive one isn't seen. For a person to see a recessive trait, the
person must have 2 recessive genes in the gene pair
Homozygous- Both alleles are identical. AA, tt, yy, BB, nn...... “purebred”
Heterozygous- Both alleles are different. Aa, Tt, Yy, Bb, Nn.......
The dominant allele wins out & is shown. “hybrid” or “carrier”
Genotype- The 2 letters a person has for a trait. Examples: AA, Tt, rr
Phenotype- What the trait looks like (blue eyes, brown eyes, etc....)
Gregor Mendel – “father of genetics” an Austrian monk, was the first to succeed in predicting how traits are
carried from one generation to the next. He used pea plants in his experiments because they reproduce sexually. He was
very careful to study one trait at a time to control the variables.
Monohybrid Cross: genetic cross that involves one trait
Punnett Square: device used to predict the probability of possible offspring from a cross.
MONOHYBRID CROSS
Ex. In rabbits, black fur (B) is dominant over brown fur (b). If one parent rabbit is heterozygous (Bb) and the other parent
rabbit is homozygous brown (bb), what is the probability of producing an offspring with brown fur? Use the Punnett
square to determine your answer.
For this cross, the Punnett square would look like this:
Half (50%) ofthe offspring would be black (Bb)
Half (50%) would be brown (bb).
The probability of an offspring with brown fur is 50%, or 2 out of 4.
Mendel’s work can be summarized in three laws:
 Law of Dominance the dominant allele will prevent the recessive allele from being expressed. The recessive
allele will appear when it is paired with another recessive allele in the offspring.
 Law of Segregation (separation) gene pairs separate when gametes are formed, so each gamete (sex cell) has
only one allele of each pair.
 Law of Independent Assortment different pairs of genes separate independently of each other when gametes are
formed.
Codominance:
2 types of alleles are equally dominant.
Heterozygous = phenotype is a blending, because both traits are expressed equally. MIXED
Ex. Red Flower (RR) White Flower (WW)
Pink Flower (RW)
Incomplete Dominance
Neither allele is completely dominant
Heterozygous = phenotype shows BOTH traits
Ex. Red Cow (RR)
White cow (WW)
Red and white cow “roan” (RW)
Sex- Chromosomes
In humans, the 23rd pair of chromosomes determines the gender. (X and Y)
The X chromosome is larger and carries more genes than the Y chromosome
XX = female.
XY = male.
Sex-linked trait: a gene is found only on the X chromosome and not the Y chromosome
Ex. Hemophilia is a sex-linked trait. If a female carrier for hemophilia is crossed with a male with hemophilia, what is the
probability that the offspring will suffer from the disease?
H=normal and h=hemophilia
50% of the female offspring will be carriers of hemophilia
50% of the female offspring will suffer from hemophilia
50% of the male offspring will be normal
50% of the male offspring will have hemophilia
Multiple Alleles
Traits that are the result of 2 or more alleles.
Ex. Blood type (IA , IB , i)
Genotypes:
A
B
O
Phenotypes:
IAIA or IAi
IBIB or IBi
ii
Polygenic Traits
Traits that are controlled by more than one gene. Results in a variety of Phenotypes.
Ex. Hair color, skin color, eye color
(AaBb or GGTt)
Pedigrees
Charts used to study the inheritance of a trait within a family, or within a closely related population.
Square = Male
Shaded in = affected
Circle = Female
Unshaded = unaffected
Dominant trait- seen in every generation
Recessive trait- skips generations
Sex-linked – mostly in males
Genetic Disorders
Down Syndrome (trisomy 21): an extra chromosome on the 21st pair. Due to non-disjunction.
Cystic Fibrosis: recessive lung disorder
Achondroplasia: dominant dwarfism. Due to mutation.
Huntington’s Disease: dominant nerve disorder
Sickle Cell Anemia: codominant blood disorder
Hemophilia – sex-linked blood disorder
Color-blindness – sex-linked vision disorder
DIHYBRID CROSS
Genetic cross between 2 traits.
Ex. TtBb x TtBb
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