Download CP-Ch10-MendelianGenetics

DO NOW: Pass it on!
• How are the members of this family different? How
are they the same?
• Do children ever look exactly the same?
Mendelian Genetics
• Genetics
– Branch of biology dealing with heredity and
variation of organisms
• Heredity
– Passing of traits from parent to offspring
• Chromosomes
– Carry the hereditary information (genes)
• Gene
– Arrangement of nucleotides in DNA
– DNA --> RNA --> proteins
Genetics
• Gene (trait)
•
•
•
•
Hair color
Eye color
Height
Skin color
• Allele
– Different forms of a gene
Allele
• Different forms of a gene
– found at the same position
on homologous
chromosomes
• Example:
– Dark purple = brown eyes
– Light purple = blue eyes
Homologous Chromosomes
*Sister
chromatids are
identical,
*homologous
chromosomes
are not
• Chromosomes (w/genes) occur in pairs
• Colors represent different alleles
NEED TO KNOW: Terminology
• Locus
– Fixed location on DNA where gene is found
– Found at same location on chromosome
Gregor Mendel
(Father of genetics)
• “pure” lines of peas
• Studied one trait through
several generations
• Used probability
• Past: Inherited traits thought to be a
“dilution” or “blend” of parental traits
• Mendel predicted
– Genes “factors” occur in pairs
– One gene of each pair is present in gametes
Mendel’s Peas
• Looked at seven
traits of peas
• To study one trait Mendel…
– Crossed two plants with different alleles of that one trait
• Height: tall x short
– Found that the offspring looked like either parents
• Offspring:
• He found dominant and recessive traits
NEED TO KNOW: Terminology
• Homozygous
– Having identical genes for a certain trait (one
from each parent)
• Homozygous dominant (TT)
• Homozygous recessive (tt)
• Heterozygous
– Having two different genes for a certain trait
(Tt)
• Dark blue (BB)- brown
eyes
• Light blue (bb)- blue eyes
• During sexual
reproduction, crossing over
occurs and makes new
combinations of genes
• (BB, Bb, bb)
What heights would your plants be
if you…
• Crossed two pea plants that differ only
in height (one tall and one short)
– Parent pea plant 1: Tall (T)
– Parent pea plant 2: Short (t)
Monohybrid Cross
(one trait difference)
• T = allele for tall
• t = allele for short
• TT = homozygous tall plant
• Tt = heterozygous tall plant
• tt = homozygous short plant
NEED TO KNOW: Terminology
• Dominant
– Allele of a gene that masks the expression of an
alternate allele
– Appears in homozygous dominant
– Appears in heterozygote
• Recessive
– Allele that is masked by dominant allele
– Does not appear in heterozygote (Bb) (It’s carried)
– Only appears in homozygous recessive
NEED TO KNOW: Terminology
• Genotype
– Genetic makeup of an organism
– BB, Bb, bb
• Phenotype
– Physical appearance of an organism
– Description (brown eyes, blue eyes)
Punnett Square
• Tool to do genetic crosses
• For monohybrid crosses use a square
with four boxes
• Used to predict
genotypes and
phenotypes of
offspring
Using a Punnett Square steps:
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•
•
•
•
1. Determine genotypes of parents
2. Choose letters to represent genotypes
3. Write down your cross using those letters
4. Draw the Punnett Square
5. “split” the letters of the genotype and put
them outside the square
Let’s do one!
Monohybrid cross Parental generation
• Cross a homozygous tall plant with a
homozygous recessive plant
• TT x tt
T
T
t
Tt
Tt
t
Tt
Tt
TT x tt
• P=?
– True breeding homozygous (TT)
– Homozygous recessive (tt)
• F1 = ?
– ALL heterozygous (Tt)
Generations
• P = parental
• F1 = first filial
• offspring of parental cross
• F2 = second filial
• offspring of F1 generation cross
Now an F1 generation cross
• Cross the offspring of your F1 generation
T
t
T
TT
Tt
t
Tt
tt
Results: F2 generation
T
t
T
TT
Tt
t
Tt
tt
Genotypic ratio = 1:2:1
Phenotypic ratio = 3:1
• Genotypes
– 1 TT = Tall
– 2 Tt = Tall
– 1 tt = short
• Phenotypes
– 3 Tall
– 1 short
Practice Problems!
• Worksheets!
– In-class and homework!
Mendelian Principles
• Dominance
– One allele masks another
• Segregation
– When gametes are formed, the pairs of genes
get separated
– Each gamete receives only one of each gene
• Happens when sister chromatids seperate
Segregation
Mendelian Principles
• Independent Assortment
– Genes of individual characteristics are not
connected
• Depends how chromos line up in metaphase
• Brown eyes not always inherited with long hair
• Can have brown eyes with short hair
– Advantage of sexual reproduction
Independent Assortment
Independent
Assortment
Dihybrid crosses
• Matings involving parents that differ in
two genes (two independent traits)
Chalkboard Dihybrid crosses
• Learn short cut
Test Cross!
• Individual with an unknown genotype
crossed with a homozygous recessive
individual
• Ex: You have a black guinea pig
– Is it BB or Bb?
– Need to know offspring to figure out
parents
Test Cross
• Practice problems…
Non-Mendelian Genetics
• Covers genes that do not follow
Mendel’s inheritance patterns
– 1. Incomplete dominance
– 2. Codominance
– 3. Multiple Alleles
Incomplete dominance
• Neither allele shows complete dominance
• The heterozygote is an intermediate
phenotype
• Japanese four-o’clock flowers
Incomplete
Dominance
1:2:1
CRCR : CRCW : CWCW
Red : pink : white
Codominance
• Both alleles are equally dominant
• Both alleles are expressed in heterozygote
• Example: Roan coat in cattle
– Homozygous red x homozygous white
Codominance
• Roan coat
– Each hair is neither all red or all white
How to represent
Incomplete or Codominance
• Capital letters with superscripts
– CR = red coat
– CW = white coat
– Homozygous red = CR CR
– Heterozygous = CR CW (roan)
– Homozygous white = CW CW
Multiple Alleles
• So far we have studied patterns of heredity in which
each trait has two alleles
– Pod color
• Green (G)
• Yellow (g)
• Multiple alleles- traits controlled by more than two
alleles
Multiple alleles
• ABO blood groups
IA = allele for type A
IB = allele for type B
i = allele for type O
Blood is also an
example of…?
Antigen- molecule that binds specifically to an antibody
Antibody- used to identify and combat foreign objects
Modern Genetics
• 1. Polygenic (multiple gene) inheritance
• 2. Sex-linked traits
Polygenic Inheritance
• More than one set of alleles
• Example:
– Skin color is controlled by a two allele sets
– Eye color also
Polygenic Inheritance
• If skin color was controlled by 3 gene
pairs…
– Dominant A, B, C produced pigment
– But A, B, C are incompletely dominant to a, b, c
Polygenic Inheritance
• The quantity of dominant genes
determines how much pigment
– AABBCC = lots of pigment
– aabbcc = very little pigment
– AaBbCc = middle range of pigment
– So 2 heterozygotes (AaBbCc) could
produce a child with any pigment range
Polygenic Inheritance
Polygenic Inheritance
Sex-linked traits
• There are alleles on the sex chromosomes
• Called sex-linked traits
– Y-linked
– X-linked
Y-linked
• If gene is on the Y
chromosome
– Sons will inherit it from
dad
• Hairy ears
– Not very common
– Females cannot get Ylinked traits
• Why not?
X-linked
• If gene is on X chromosome
– Can be inherited from mom or dad
– Son will ALWAYS inherit it from mom
• Why?
• Examples:
– Colorblindness
– Hemophilia
– Severe Combined Immunodeficiency
(SCID) (Boy in the Bubble Disease)
Hemophilia
• XHXH = normal female
• XHXh = female carrier
• XhXh = female with hemophilia
• XHY = normal male
• XhY = male with hemophilia
Pleiotrophy
• 1 gene affects 2 characteristics
• Sickle cell Anemia
– Affects ability of blood to carry oxygen
– Disease only present in homozygous recessive
– Affects ability of malaria to affect blood cells
– Have allele for sickle cell you can’t get malaria
Sickle Cell Anemia
• S = normal allele
s = sickle allele
• SS = normal (dies from malaria)
• Ss = sickle cell trait (protected)
• ss = sickle cell anemia (dies from sickle
cell anemia)
Epistasis
• Two genes affect 1 trait (masking)
• Labrador fur
– B = black
– E = enzyme
b = brown
e = no enzyme
• BBEE = Black
Bbee = yellow
BBEe = Black
• BbEE = Black
Bbee = yellow
BbEe = black
• bbEE = brown bbee = yellow
bbEe = brown
Linked genes
• Two genes for two
different traits on the
same chromosome
• Normally inherited
together
– Light hair/light eyes
– Red hair/freckles
Linked genes
• The closer the two traits are on the
chromosome, the more likely they will
be passed on together
• When they are far apart and become
separated this is called…
• CROSSING OVER
Crossing over
• During Meiosis when
sister chromosomes
are lined up they
exchange genetic
information
Importance…
• Sex-cell mutations
– Important!
– If mutation is present in gamete, when
fertilization occurs, the embryo has
mutation
Why is learning about mutations
important?
• Lead to recessive/dominant,
autosomal/sex-linked disorders!
– Hemophilia
– Cystic fibrosis
– Down syndrome
– Tay-Sachs disease
– Huntington disease
– Sickle Cell
Testing for genetic disorders
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Karyotyping
Amniocentesis
Chorionic villus sampling
Ultrasound
Fetoscope
• Sometimes genes “jump” to new
locations on the same chromosome
• Can cause inactivation of gene
• Important sources of variation between
species
Jumping
genes