Download Chapter 11 Introduction to Genetics

Chapter 11
Introduction to
Genetics
Gregor Mendel
A brief history
• Gregor Mendel
– Breeding
experiments
using garden
peas (1850s)
– Established
basic
principles of
heredity
– Father of
genetics
Basic vocabulary
• 1) Dominant
– Trait that is
expressed (by
covering another)
• 2) Recessive
– Trait that is hidden
(may be covered by
another)
• 3) Alleles
– All genes for a
specific trait
Dominant and recessive traits
• The traits that seem to mask other traits
when present are called dominant traits.
• The traits that seem to be hidden in the
presence of dominant traits are called
recessive traits.
• 4) Homozygous (pure)
– Both genes for a trait are the same
• 5) Heterozygous (hybrid)
– Both genes for a trait are different
Homozygous vs. Heterozygous
• Homozygous: inheriting two of the same
alleles for a trait (AA tall/tall or aa
short/short)
– Allele: a variety of a trait
• Heterozygous: inheriting different allele
for a trait (Aa, tall/short)
• Mendel concluded that each trait has two
alleles that determines its appearance.
• 6) Phenotype
• Actual expression of a gene (words
describing)
• 7) Genotype
• Actual genes present (symbols used)
Phenotype?
Red or White
Genotype?
RR or Rr or rr
• 8) Mendel’s crosses:
– P = original parents
– F1 = first filial (offspring) of parents
– F2 = second filial (offspring of F1)
What did Mendel observe?
• When a true-breeding (homozygous) tall
plant is crossed with a true-breeding short
plant in the P generation, the F1 height trait
is always predictable. 100% are tall
plants.
P generation
F1
F2
Law of Segregation
• Mendel concluded only one allele is
passed from parent to offspring for each
trait.
• F1 plants must be heterozygous because
the P generation only passed on one tall
allele and one short allele.
• The F1 plant will then pass on to its
offspring either a tall or a short allele,
never both.
What happens when the F1 tall
plants are crossed together?
• Mendel observed that the F2 generation,
the offspring of F1 plants, are always in a
fixed ratio of 3:1 tall:short.
• Why?
P generation
F1
F2
• 9) Punnett
Squares
– Mathematical grid
showing probable
or possible
offspring from a
cross (phenotype &
genotype ratios)
– Monohybrid cross =
working with only
one trait
Punnett square Tt x Tt
• Punnett created a
table to show the
average
inheritance pattern
of one offspring.
Pea traits that Mendel identified
• Through multiple crosses, Mendel
determined that all these traits displayed a
mathematical predictability for inheritance.
Seed Coat
Color
Pod
Shape
Pod
Color
Smooth
Green
Seed
Shape
Seed
Color
Round
Yellow
Gray
Wrinkled
Green
White
Constricted
Round
Yellow
Gray
Smooth
Flower
Position
Plant
Height
Axial
Tall
Yellow
Terminal
Short
Green
Axial
Tall
Law of independent assortment
• Because organisms are made up of more
than one trait, Mendel concluded that the
inheritance of one trait does not influence
the inheritance of a second trait.
• Example: Height of the pea plant does not
influence the color of the peas
– Height is independently assorted from color.
10) Dihybrid cross
– working with two traits (gives twice as many
gametes possibilities, so 4 times as many
offspring) classical ratio of 9:3:3:1
Using dihybrid crosses to show
independent assortment
• A smooth, yellow
pea (RrYy) can
pass on these
combinations of
genes to its
offspring: RY, Ry,
rY, or ry.
• 11) incomplete dominance
– Both alleles for a trait blend together creating
a new expression in the heterozygous
condition
– examples:
snapdragons
Variations on Mendel
• Incomplete
dominance: the
heterozygous
genotype shows a
blend of the two
parents and not the
dominant allele
• 12) co-dominance
– Both alleles for a trait show up equally
– Examples: roans, “checkered” chickens
Variations on Mendel
• Codominance: the
heterozygous genotype
shows both inherited
alleles
• Example of roan horse
coat: AA (dark red) x aa
(white)  Aa (dark red
and white)
• 13) multiple alleles
– More than two alleles for a trait
– Examples: coat color of rabbits
Key
C = full color
CC, Ccch, Cch, or Cc
cchch, cchcch, or cchc
Cch = chinchilla
Ch = Himalayan
C = albino
chc or chch
cc
Variations on Mendel
• Multiple alleles: when
there are more than two
alleles that code for a
trait
• Example: ABO blood
type
A type = AA or Ao
B type = BB or Bo
O type = oo
AB type = AB
Blood typing
• 14) polygenic
inheritance
– Many genes
affect the
expression of
the trait
– Examples:
skin, eye, &
hair colors
Variations on Mendel
• Polygenic trait: when more than one gene
codes for a particular trait
– Example: fur color, human height, human skin color,
eye color
Modernizing Mendelian genetics
• DNA is the basis for
inheritance.
• DNA are coiled into
chromosomes.
• Parts of the DNA that
code for a trait are called
genes.
• Some genes have only
two alleles and other
have more.
Gene for
hairline
Allele: A
Genotype: Aa
Gene for
hairline
Allele: a
• Homologous
Chromosomes.
(homologues)
– Chromosome
pairs or mates
(similar in size,
shape, info)
– 0ne from each
parent
– Humans have 23
pair (total = 46)
Meiosis
• A process of reduction division in which
the number of chromosomes per cell is cut
in half through the separation of
homologous chromosomes in a diploid
cell.
• Meiosis usually involves two distinct
divisions, called meiosis I and meiosis II.
By the end of meiosis II, the diploid (2n)
cell that entered meiosis has become four
haploid (n) cells.
Meiosis
• Production of haploid gametes
• Similarities to Mitosis:
– Same basic stages, except occur twice
• Gametes
– Reproductive cells (eggs & sperm)
• Haploid (n or 1n)
– A cell with one of each type of chromosome or
half the total number
• Result is “reduction division” into 4 haploid
cells instead of 2 diploids
– In males = all 4 become sperm
– In females = only 1 egg; other “polar bodies”
disintegrate
Meiosis 1
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two
The chromosomes line up in a The sister chromatids
haploid (N) daughter cells,
similar way to the metaphase separate and move toward
each with half the number of stage of mitosis.
opposite ends of the cell.
chromosomes as the original.
Telophase II
Meiosis II results in four
haploid (N) daughter cells.
• Zygote
– (fertilized egg) first
cell of an organism
with a complete set
of chromosomes
• Diploid (2n)
– Cell with a full set
(two of each type) of
chromosomes
Oocyte or Spermatocyte
(egg cell) (sperm cell)
• This cell that can
undergo meiosis
originally has 6
chromosomes and
has replicated to 12
chromosomes in
preparation for
meiosis.
• Differences:
– Homologues
remain together
during prophase I
– Crossing over
(when chromatids
exchange pieces)
occurs during
metaphase I
Click on image to play video.
Prophase I: homologous
chromosomes pairing into tetrads
Metaphase I: tetrads align, along
the metaphase plate
Anaphase I: homologous chromosomes
separate from the metaphase plate
• If chromosomes
do not properly
separate, this is
called
nondisjunction.
• Nondisjunction
leads to trisomy
and monosomy
disorders.
Telophase I: membranes form
around the separated homologues
Prophase II: spindle fibers bind to the
sister chromatids of each chromosome
Metaphase II: chromosomes align
along the metaphase plate
Anaphase II: sister chromatids
separate to opposite poles
Telophase II: nuclear membrane forms
around newly separated chromatids
• Note that each new nucleus formed has ½
the amount of DNA as the original cell.
• These cells are haploid cells.
Nondisjunction
Click on image to play video.
Click on image to play video.
How can siblings look alike but not
exactly the same if they come from the
same parents?
Crossing over
• The chromosomes during prophase I
undergo crossing over, where parts of the
homologues randomly switch places.
Importance of crossing over
• The gene
combinations that a
person gets from his
or her parents will be
different, to varying
degrees, than the
combination a sibling
may get.
More sibling similarities
Variations on Mendel
• Linked genes: Mendel concluded that
traits are assorted independently, but
some traits are linked.
• This means that two genes are almost
always inherited together (ex: red hair,
green eyes).
Fruit fly chromosome #2
Exact location on chromosomes
Chromosome 2
For example, a fly with reddish-orange eyes and
miniature wings, were almost always linked, inherited,
together.