ap biology inheritance and chromosomal mutations
TRANSCRIPT
Warm up Match the items on the left with one item on
the right1. HH A. heterozygous2. Curly hair B. homozygous3. Hh C. phenotype4. Genotype D. tt
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Crash Course Biology Hank Green
Bozeman Biology
Paul Anderson
Inheritance
Ch. 11
Main Topics Gregor Mendel’s work Mendel’s Laws Dominant/recessive Heterozygous/homozygous Alleles Codominance and incomplete dominance Epistasis, Pleiotropy, Multifactorial
Inheritance, Polygenic Traits
The father of genetics
Gregor Mendel is considered the Father of Genetics
Born in 1822 Studied math &
physics at an Austrian university
He was the first person to study how traits are passed along from one generation to the next.
He did his work with the pea plant
Who’s your
daddy?
Mendel’s Garden
Analyzed observable traits of peas growing in his monastery garden.
Mendel’s Garden
Eight years & 20 volumes of data and analysis on 7 distinctive traits
Published in 1865
Why peas?
The garden pea was a good choice for a variety of reasons. The garden pea: is easy to raise produces large numbers of offspring reproduces quickly has flowers which are self fertilizing
but can be easily crossed to other varieties
Experimental Approach
Can also be cross-fertilized by human manipulation
Mendel cross-fertilized true-breeding garden pea plants having clearly contrasting traits
Allele for purple flowers
Homologouspair ofchromosomes
Allele for white flowers
Locus for flower-color gene
Mendel's Theory of Segregation
Diploid organisms inherit two genes per trait
Each gene segregates from the other during meiosis so that each gamete will receive only one gene per trait
How can the Chances of an Offspring’s Traits be Determined?
The chance of an offspring showing a certain trait can be determined by using the Punnett square.
The table contains spaces for the parent’s gametes and the possible offspring from that mating.
The alleles are represented by their letters. Genes come in pairs and must be separated during
gamete formation. These gametes (letter) of each pair are placed in
each of the outside spaces. They are then combined to form the possible
offspring.
Punnett Square: Bb X Bb
bbBbb
BbBBB
bBGametes
Monohybrid Crosses
Mendel's first experiments
One trait Monohybrid crosses
have two parents that are true-breeding for contrasting forms of a trait.
All the offspring from the first cross showed only 1 form of the trait
This trait seemed “stronger” so he called it DOMINANT
When he crossed the offspring from the first cross, the other form of the trait reappeared, but only 1/4 of the time
This trait seemed “weaker” so he called it recessive
Predicting the OutcomeWhy does one form of the trait
disappear in the first generation (F1 ),
only to show up in the second generation (F2 )??
Artificial selection: populations could evolve (i.e. change) if members show variation in heritable traits
Variations that improved survival chances in the wild would be more common in each generation
This idea is known as natural selection
Prevailing Theories
Mendel’s Experiments Natural selection did not fit with
prevailing view of inheritance-blending Blending would produce uniform
populations; such populations could not evolve
Mendel’s Experiments Many observations did not fit
blending A white horse and a black horse did
not produce only gray horses
Test (Back) Crosses To support his concept of
segregation, Mendel crossed F1 plants (Pp) BACK with homozygous recessives (pp)
What ratio would Mendel have gotten?He didn’t know the letter
combination of the F1 plants. The test (back) cross allowed him to figure it out
Dominant phenotype,unknown genotype:
PP or Pp?
If PP,then all offspring
purple:
p p
P
P
Pp Pp
Pp Pp
If Pp,then 1
2 offspring purpleand 1
2 offspring white:
p p
P
Ppp pp
Pp Pp
Recessive phenotype,known genotype:
pp
His back crossed supported his idea of 2 “factors” for each individual, and the idea that those “factors” are segregated
Dihybrid Crosses
Mendel also performed experiments involving two traits
Predicting the OutcomeWhat is the predicted
PHENOTYPIC ratio
and the predicted
GENOTYPIC ratio that Mendel saw?
Predicting the Outcome The F2 results showed
9/16 were tall and purple-flowered and 1/16 were dwarf and white-flowered-as were the original parents; however, there were 3/16 each of two new combinations: dwarf purple-flowered and tall white-flowered.
OutcomesMonohybrid crosses
Both parents HETEROZYGOUS3:1 phenotype
Dihybrid crossesBoth parents HETEROZYGOUS
9:3:3:1 phenotype
Theory of Independent Assortment
Each gene of a pair tends to assort into gametes independently of other gene pairs on non-homologous chromosomes
Theory in Modern Form
Genes located on non-homologous chromosomes segregate independently of each other
Practice with your neighbor
For the following questions Work with your neighbor to answer
the question. Answer the multiple choice
questionthen,
Use your notes to determine which one of Mendel’s principles it demonstrates
1. A father carries 2 alleles for the gene for widow’s peak. He
carries one dominant allele and one recessive allele. His
gametes willa. All contain the dominant alleleb. All contain the recessive allelec. ½ will get the dominant allele and ½ will get
the recessive alleled. Each gamete will get both the dominant and
the recessive allele
Which principle does question number one best
demonstrate? Principle of
Segregation
The dominant allele goes to one gamete and the recessive allele goes to another gamete
2. A mother that is homozygous dominant for bushy eyebrows (BB) and heterozygous for round ears (Rr). The gametes she can make
will a. All have a B and a R in themb. ½ will have a B and ½ will have a R or
a r in themc. ½ will have a B and a R and ½ will
have b and rd. ½ will have B and R and ½ will have B
and r
What principle does number 2 demonstrate?
The Principle of Independent Assortment
All gametes will have a B, since mom only has B.
The big B can be with the big R or the big B can be with the little r.
3. In meiosis, a diploid cell divides twice to form 4 haploid
gametes. Each gamete contains:
a. A complete set of DNA identical to the parents
b. A ½ set of DNA, with just one copy of each chromosome
c. Homologous pairs of chromosomesd. Multiple copies of chromosomes,
depending on which ones moved during meiosis
Which one of Mendel’s Principles does number 3
demonstrate? Principle of Segregation
All the homologous pairs of chromosomes separate so that there is just one of each pair in each gamete.
4. When Mendel crossed a true breeding green pea plant (GG) with a true breeding yellow pea plant (gg), the offspring plants
werea. All greenb. All yellowc. ½ green and ½ yellowd. Green and yellow mixed
Which one of Mendel’s principles does number 4
demonstrate? Principle of Complete Dominance
All offspring were Gg, and the dominant allele (G) masked the recessive allele (g)
5. Mendel wanted to know if the color for pea seeds was linked to the shape of the pea seeds. He crossed a green, wrinkled seed
plant (Ggrr) with a yellow, smooth seed (ggRr) plant. The offspring
produced were:a. All green and wrinkledb. All yellow and wrinkledc. All green and smoothd. All yellow and smoothe. Some of each of the above
Which one of Mendel’s Principles does number 5
demonstrate? Principle of Independent
Assortment
The green trait can go with the smooth or the wrinkled trait
The yellow trait can go with the smooth or the wrinkled trait
Mendel’s Work
The work that Mendel did helped explain patterns of inheritance in eukaryotes.
But Mendel worked with traits that had a clear dominant/recessive pattern.
Also, the traits he worked with were all controlled by a single gene.
Different Patterns of Inheritance As we now know,
many traits do not follow Mendelian Inheritance patterns.
Degrees of Dominance Complete Dominance - BB and Bb =
same phenotype Incomplete Dominance - Bb has in-
between phenotype Codominance - Bb has both B and b
phenotype
Co-dominance When both
alleles are expressed equally in the heterozygous individual.
A and B blood type alleles are co-dominant, because a person with AB genotype will have both A and B blood proteins.
Black and orange color in cats are co-dominant, because a heterozygous female will have both orange and black hair.
Incomplete Dominance Both alleles are
blended together in the heterozygous individual.
Dominant allele cannot completely mask the expression of another
Multiple Alleles
More than 2 versions (alleles) for a single trait
can be completely dominant or codominant
Blood TypesGenotype of offspring
Phenotype of offspring
A
iAiB AB
iAi A
iAiA
iBiB B
iBi Bii o
Rh factorRh factor Possible genotypes
Rh+
Rh-
+/+ or +/-
-/-
So far we’ve only looked at how a single gene pair affects phenotype
More often - multiple genes involved 2 primary cases:
1. 2 or more genes affect a single trait 2. 1 gene affects the phenotype of
another gene
Epistasis (standing upon)- 2 or more genes affect a single
trait Labs can be black, yellow, or chocolate
Black is dominant to chocolate
BB and Bb = black bb = chocolate
AND - another gene P codes for whether or not any pigment is put into the hair
PP and Pp = hair has pigment and dog will be black (BB or Bb) or brown (bb)
pp = no hair pigment and dog will be yellow, regardless of the “b” alleles
So in this case, the P gene “stands upon” the B gene
P is epistatic to B We don’t get the classic 9:3:3:1 but some
other version of it
Pleiotropy A single gene can
have multiple effects on phenotype
e.g. pleiotropic alleles --> multiple symptoms of sickle cell anemia (pain, jaundice, infections, fatigue, etc)
Polygenic Inheritance 2 or more genes affect a single
phenotypic trait Eye color, skin color, height
Skin color is controlled by at least 3 separate gene pairs
Genotype AABBCC would be very dark skin
Genotype aabbcc would be very light skin Any other combination would be
intermediate
And, of course, skin color is also influenced by your environment - multifactorial inheritance
X-linked traits genes found on the X
chromosome. show different
inheritance patterns in men than in women.
X-linked traits may show dominant/recessive or codominant patterns.
Sex-linked genes
• An organism’s sex is an inherited phenotypic character determined by the presence or absence of certain chromosomes
• Mammals like humans have an XX or XY system of inheritance
• Other organisms have other systems
Genes on the sex chromosomes are called
sex-linked genes• Some diseases on the X
chromosome:• Color blindness
• Rare in females, mild disease• Duchenne muscular dystrophy
• 1 in 3500 males in US gets it• Lack the gene for the muscle protein
dystrophin• Muscles get weaker and lose
coordination• Usually don’t live past 20s
• Hemophilia• Lack the protein to cause clotting• Don’t clot normally
Barr bodies
• In mammalian females, 1 of the 2 X chromosomes is inactivated during embryonic development
• The inactive X condenses into what is called a Barr body (we can see it under the microscope)
• If she is heterozygous for a sex-linked trait, she will be a mosaic for that trait
• Some cells have the maternal X inactivated• These cells have
the orange color• Some cells have
the paternal X inactivated• These cells have
the black color• All cells in the
ovaries have active X chromosomes
Y-linked traits Y-linked traits called holandric
inheritance. Y-chromosome is small and does not
contain many genes Deletions on y chromosome male
infertility SRY gene sex determining region
The curious case of the guevedoces
Deficient in an enzyme that converts testosterone to dihydrogen testosterone, so don’t develop male genitalia as embryos.
20.Orange and black coat color are on the X
chromosome in cats and they are codominant to each other. Tortoise shell is the codominant phenotype.
A black female (XBXB) mated with an unknown male. The kittens were:
2 tortoise shell females and 2 black males.What is the father’s genotype and phenotype?
XOY- orange
21.• Ricket’s is a dominant disorder on the X
chromosome in humans. • X = normal XR = affected by ricketsA couple wants to know their chances of having
a child born with Rickets.The wife is normal, the husband has the
disease.What are the chances of having an affected
son? An affected daughter?
0% affected son, 100% affected daughter
22.• Another couple, same disease. This
time, the wife is affected. Her father was normal. The husband is not affected. Same question: chances of an affected son? Affected daughter?
50% son, 50% daughter
23.• A tortoise shell female mated with an
unknown male. The kittens were 2 orange females, 1 tortoise shell females, 1 black male, 2 orange males.
• What is the genotype and phenotype of the father?
XOY- orange
Chromosomal mutations• In nondisjunction,
pairs of homologous chromosomes do not separate normally during meiosis
• As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy
What results…• Aneuploidy - a zygote
produced from a normal gamete and a gamete produced by nondisjunction • Offspring with this
condition have an abnormal number of a particular chromosome
What results…
• Trisomy - having 3 copies of a particular chromosome• Monosomy - having just one copy of a particular
chromosome• Polyploidy - a condition in which an organism has more
than two complete sets of chromosomes
Recent research has shown that this Chilean rodent is a tetraploid
Very rare among animals
Common in plants, some fish, some amphibians
Chromosomal breakage• Breakage of a chromosome can lead to
four types of changes in chromosome structure:• Deletion removes a chromosomal
segment
Deletion mutation• Example: retinoblastoma (eye
tumors)
Chromosomal breakage• Duplication repeats a segment
Duplication mutation – fragile X syndrome
Chromosomal breakage• Inversion reverses a segment within a
chromosome
Hemophilia A – inversion mutation patient was given
injection in buttocks
Chromosomal breakage• Translocation moves a segment from
one chromosome to another
Translocation mutation causes Burkitt’s lymphoma Tumors on
hand from cancer
Why does this happen? When would you predict these kinds of
chromosomal errors would occur?
Why?
Down syndrome• Trisomy 21 - 3 number 21
chromosomes• 1 in 700 children in US• Frequency increases with
age of mother
Trisomy 18 – Edward’s syndrome low birth weight,
mental retardation, extra fingers and toes
Trisomy of sex chromosomes• Klinefelter
syndrome is the result of an extra chromosome in a male, producing XXY individuals
• Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans
DNA is also found in mitochondria and chloroplasts.
Mitochondrial DNA is only passed from Mother to child.
How are traits inherited?
What mode(s) of inheritance would you predict for the trait of skin color? Why?
Make a list of all the possible modes of inheritance we’ve learned about
Next to each one give a short definition and an example