Transcript Patterns of Inheritance

Patterns of Inheritance
Chapter 10
Genetics developed from curiosity
about inheritance
• Gregor Mendel
– Late 1800s
– Pea plants
Genetics developed from curiosity
about inheritance
• Trait= a variation of a particular character
• Genetics= study of heredity
• Self-fertilization= fertilization of an ovum
of a plant by a male gamete from the
same flower
– True breeding
• Cross-fertilization= sperm from the
pollen of one flower fertilizes the eggs in
the flower of a different plant
Mendel discovered that inheritance
follows rules of chance
• Hybrids= the offspring of two different truebreeding varieties
– P generation= P for parent
– F1 generation= F for filial (Latin for son)
– F2 generation= offspring of the F1
generation…“grandchildren”
• Monohybrid cross= a pairing in which the
parent plants differ in only one (mono) character
– Mendel crossed purple-flowered pea plants with
white-flowered pea plants
Mendel discovered that inheritance
follows rules of chance
Mendel discovered that inheritance
follows rules of chance
• Mendel not only used flower color, he also
used flower position, seed color, seed
shape, pod shape, pod color, and stem
length
• He used monohybrid crosses to study the
heredity of these traits
Mendel discovered that inheritance
follows rules of chance
•
Mendel’s 4 hypotheses:
1. There are other forms of genes.
– Such as the gene for flower color
– One form for purple flowers
– Another for white flowers
– Alternative forms of genes are called alleles
2. For each inherited trait, an organism has two
alleles for the gene controlling that
trait…one from mom and one from dad.
– Heterozygous= two alleles for a trait are
different
– Homozygous= two alleles for a trait are the
same
Mendel discovered that inheritance
follows rules of chance
3. When only one of the two different alleles appears
to affect the trait, that allele is called the dominant
allele. The other allele that does not appear is
called the recessive allele.
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Flower color- Purple
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P = dominant
p= recessive
4. The two alleles for a trait separate during the
formation of gametes (remember…meiosis!). Each
gamete has only one allele for each trait. The union
of sex cells, during fertilization, reunites the alleles
to form pairs again.
Mendel discovered that inheritance
follows rules of chance
• You can calculate the probabilities for
different combinations of alleles resulting
from a genetic cross
• Punnett square= a diagram that shows
all possible outcomes of a genetic cross
Mrs. B’s QUICK
QUIZ
1. The father of genetics is
Friar Tuck
Gregor Mendel
Anton van Leeuwenhoek
Bill Nye
Gregor Mendel
2. _____________ is the fertilization of an
ovum of a plant by a male gamete from
the same flower.
True breeding
Cross fertilization
Self fertilization
Genetics
Self fertilization
3. A hybrids is the offspring of one true
breeding plant.
True
False
False
Hybrids= the offspring of two different truebreeding varieties
Mendel discovered that inheritance
follows rules of chance
• The genetic makeup , or combination of alleles is called
the genotype
– The genotypic ratio is 1PP:2Pp:1pp
• The observable trait is called the phenotype (purple
flowers)
– The 3:1 ratio is called a phenotypic ratio
Mendel discovered that inheritance
follows rules of chance
• Testcross= breeds an individual of
unknown genotype, but dominant
phenotype with a homozygous recessive
individual
• The appearance of the phenotype of the
offspring will reveal the mystery plant
Mendel discovered that inheritance
follows rules of chance
• Dihybrid cross= crossing individuals that differ
in two characters
• Mendel used this when studying peas with
–
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–
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–
different colors (yellow =dominant)
different shapes (round= dominant)
RRYY
RrYy
Rryy
A punnett square can be used in the same way
Mendel discovered that inheritance
follows rules of chance
• Mendel used dihydrid crosses to study all seven
characteristics of pea plants
• Proposed his principle of independent
assortment, which states that during gamete
formation in an F2 cross, a particular allele for
one character can pair up with a particular allele
of another character
– R can end up with Y or y
– r can end up with Y or y
– The alleles are sorted into gametes independently of
one another
There are many variations of
inheritance patterns
• For some characters of organisms, neither
allele is dominant
• The heterozygotes have a phenotype that
is intermediate between the phenotypes to
the two parents
• This is called incomplete dominance
– Ex: Andalusian chickens
– A heterozygote chicken=CBCW
– This is NOT blending, because the parent
phenotypes can reappear in the F2 generation
There are many variations of
inheritance patterns
• Many genes have multiple alleles, instead
of just two
– Ex: human blood type
– A, B, AB, O
– IA and IB display codominance, meaning that
a heterozygote expresses both traits
– This is NOT intermediate, due to the fact that
the phenotype shows the separate traits of
both alleles
There are many variations of
inheritance patterns
• When multiple genes affect a character, the
variation in phenotypes can increase
• When two or more genes affect a single
character, it is called polygenic inheritance
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ex: height in humans
3 tall alleles: A, B, C
3 short alleles: X, Y, Z
Intermediate inheritance
AABBCC= very tall
AZBBCC= slightly shorter, etc.
• Some characters are affected by dozens of
genes, which lead to numerous combinations of
alleles
There are many variations of
inheritance patterns
• An individual’s phenotype depends on
environment as well as on genes
– Ex: tree
– Temperature
• The product of a genotype is generally a
range of possibilities influenced by the
environment
Meiosis explains Mendel’s
principles
• Chromosome theory of inheritance:
genes are located on chromosomes, and
the behavior of chromosomes during
meiosis and fertilization accounts for
inheritance
Meiosis explains Mendel’s
principles
• Gene locus
• When genes are located on separate
chromosomes, they sort independently of each
other during meiosis
• Crossing over allows genes on the same
chromosome to sort independently
• The tendency for alleles on one chromosome to
be inherited together is called genetic
linkage…the closer the 2 genes are on a
chromosome, the greater the genetic
linkage…the farther apart they are, the more
likely a cross over will occur
Sex-linked traits have unique
inheritance patterns
• Eggs contain what kind of chromosome?
X
• Sperm contain what kind of chromosome?
Half have X and half have Y
• An offspring’s sex is determined by whether or
not the sperm cell carries an X or a Y
• Any gene that is located on a sex chromosome
is called a sex-linked gene
• In humans, most are on the X chromosome
Sex-linked traits have unique
inheritance patterns
• Thomas Morgan
– Fruit flies
• Sex-linked disorders
– Red-green colorblindness
• X-linked recessive
• Recessive sex-linked traits are
much more common in men than
in women
Pedigrees
• Family tree that
records and traces
the occurrence of a
trait in a family
• Circles = females
• Squares = males
• Horizontal lines
connecting a square
to a circle means the
couple had children
Pedigrees
• If shape has
two colors the
person in a
carrier