Chapter 6 Flashcards
Father of modern genetics
Gregor Mendel (austrian monk)
What did gregor mendel do?
- Bred garden peas to study patterns of heredity
2. Statistically analyzed hundreds of plants across many generations
What did gregor mendel do?
- Bred garden peas to study patterns of heredity
2. Statistically analyzed hundreds of plants across many generations
Mendel’s work produced which laws:
Law of dominance, law of segregation, and law of independent assortment
Probability
likelihood a particular event will occur but it can’t predict whether event will actually occur
If sample is large enough
can predict an average outcome
Law of Dominance
This law states that when two organisms, homozygous (pure) for 2 opposing traits are crossed, the offspring will be hybrid (carry 2 diff alleles) but will exhibit only the dominant trait
Hidden trait is the
recessive one
Law of Segregation
State that during the formation of gametes, the 2 traits carried by each parent separate
Monohybrid cross
a cross between 2 organisms that are each hybrid for a single trait
Monohybrid cross percentages
The genotype (types of genes) ratio is 25% homozygous dominant (TT) to 50% heterozygous (Tt) to 25% homozygous recessive (tt) or 1 : 2: 1.
Backcross or Testcross
a way to determine whether an individual plant or animal showing the dominant trait is homozygous dominant (BB) or heterozygous (Bb)
Backcross/Testcross Step 1
- To determine genotype, an individual of the unknown genotype (B_) is crossed with a homozygous recessive individual (bb)
The genotype B_ means that oen allele is dominant (B) but the other is uncertain, B/B or B/b
Backcross/Testcross Step 2
- If the individual being tested is homozygous dominant (BB) → all offspring of the test cross will show the dominant trait and have a hybrid (Bb) genotype
Backcross/Testcross Step 4
- If the individual being test is actually hybrid (Bb), we can expect that ½ of the offspring, or at least 1 individual, will show the recessive trait
Therefore, if any offspring show the recessive trait, the parent of unknown genotype must be hybrid
Law of Independent Assortment
This law states that during gamete formation, the genes for one trait (such as height, T or t) are not inherited along with the genes for another trait (such as seed color, Y or y )
What does the law of independent assortment apply to?
This applies when a cross is carried out between 2 individuals that are hybrid for 2 traits on separate chromosomes
What factor determines how alleles are inherited?
how the homologous pair ( in this case TY and ty) happen to line up in metaphase of meiosis I, which is a random event
Crossing Tt Yy x Tt Yy is called
a dihybrid cross because it is a cross between individuals that are hybrid for 2 different traits
A dihybrid cross can produce
4 diff types of gametes TY, Ty, tY, and ty
Incomplete Dominance
characterized by blending
Examples of incomplete dominance
- A long watermelon (LL) crossed with a round watermelon (RR) produces all oval watermelons (RL)
- A black animal (BB) crossed with a white (WW) animal produces all gray (BW) animals
(In incomplete dominance) Since neither trait is dominant,
the convention for writing the genes uses different capital letters
Codominance
both traits show
Example of codominance
the MN blood group in humans ( not related to ABO blood group)
3 Diff blood groups: M, N, MN
these groups are based on 2 distinct molecules located on the surface of the red blood cells.
A person can be
1) homozygous for 1 type of molecule ((MM), homozygous for the other (NN), or be hybrid and have both molecules (MN) on the surface of their red blood cells.
MN gentoype is not intermediate between M and N,
both M and N are fully expressed on the surface of red blood cells
Multiple Alleles
Most genes in a population exist in only 2 allelic forms (ex. Pea plants can either be tall (T) or dwarf (t))
Example of Multiple Allele
in humans there are 4 diff blood groups depending on the presence of specific molecules on the surface of RBCs: A, B, AB, AND O
Four diff blood types determined by
3 alleles A, B, AND O.
A and B are codominant and are often written as
I ^A and I^ B ( i stands for immunoglobulin).
When both alleles are present (A/B), they are both expressed
and the person has AB blood type
O is a
recessive trait often written as i
Blood Type: A
Genotype: homozygous A:
AA
Blood Type: A
Genotype: hybrid A:
Ai
Blood Type: B
Genotype: homozygous B:
BB
Blood Type: B
Genotype: hybrid B:
Bi
Blood Type: AB
Genotype: heterozygous:
AB
Blood Type: O
Genotype: homozygous O:
ii
A person can have any one of the
6 blood genotypes
Polygenic Inheritance
Height and other similar features are controlled by not just one gene but multiple genes that each make a small contribution to the overall outcome
Many characteristics such as skin color, hair color, and height result from a
blending of several separate genes that vary along a continuum , they are controlled by several genes
Sex-Linked Genes
Traits carried on the X chromosome are called sex-linked
Females inherit
two copies of sex-linked genes (XX)
If a sex-linked trait is due to a recessive mutation, a female will
express the phenotype only if she carries 2 mutated genes (X-, X-)
If she carries only one mutated X-linked gene
she will be a carrier (X-X)
Males (XY) only inherit
one X-linked gene
If the male inherits a mutated X-linked gene (X-Y),
he will express the gene
There are dominant sex-linked traits
but recessive ones are much more common
Who suffers with sex-linked traits more?
females more than males
Common examples of recessive sex-linked traits
color blindness and hemophilia
Who are the carriers?
All daughters of affected fathers
Sons cant inherit a sex-linked trait from the father because
the son inherits the Y chromosome from the father
What is the chance a son inherits a sex-linked trait from a carrier mother?
50%
What is the carrier state for X-linked traits in males?
there is no carrier state
if a male has the gene, he will express it
It is uncommon for a female to express a sex-linked condition because
in order to be affected, she must have inherited a mutant gene from both parents
In sex-linked inheritance, the father
passes the trait to his daughters only
What can alter the expression of genes?
the environment
In fruit flies, the expression of the mutation for vestigial wings (short, shriveled wings)
can be altered by temperature
When raised in a hot environment, fruit flies that are
homozygous recessive for vestigial wings can grows almost as long as normal wings
For humans, the development of intelligence is the result of
an interaction of genetic predispositions and the influence of the environment
Inheritance can be influenced by the
sex of the individual carrying the traits.
Pattern of baldness in males is not
sex-linked but rather sex-influenced
Males and females express the gene for pattern baldness
differently
Karyotype
Type of lab procedure that analyzes the size, shape, and number of chromosomes
When do specialists prepare and photograph chromosomes for karyotypes?
during metaphase of mitosis when they are fully condensed
In human chromosomes
44 (22 pairs) autosomes and 2 sex chromosomes
Pedigree
A family tree that indicates the phenotype of 1 trait being studied for every member of a family
Why do geneticists use pedigrees?
to determine how a specific trait is inherited
Pedigrees
Female:
Male:
circle
square
Carrier state in pedigrees
Carrier state is not always shown
-if it is, it is sometimes represented by a half-shaded in shape
A shape completely shaded in if a
person exhibits the trait
Mutations
Any abnormality in the genome → can occur in somatic (body) cells and be responsible for the spontaneous development of cancer
Mutations can also occur
during gametogenesis in germ cells and affect future offspring
When and where mutations occur is
random
2 Types of mutations
Gene and Chromosome
Gene Mutations
caused by change in DNA sequence.
Chromosome mutations
can be observed under a light microscope.
A chromosome may sustain a deletion or an addition or a cell may have an entirely extra chromosome, which results from nondisjunction
Nondisjunction
An error that sometimes happens during meiosis in which homologous chromosomes fail to separate as they should
When nondisjunction occurs
1 gamete receives 2 homologues, while the other gamete recipes none → remaining chromosomes may be unaffected and normal
If either of these abnormal gametes unties with a normal gamete during fertilization
the resulting zygote will have an abnormal number of chromosomes
Any abnormal chromosome condition is known as
aneuploidy
trisomy
If the chromosome is present in triplicate
Down syndrome
3 #21 chromosomes →trisomy-21
Triploid (3n)
An organism in which the cells have an extra set of chromosomes
Examples of triploid
The cells of endosperm or cotyledon of a seed are triploid
Polyploid
An organism with more than 3 sets of chromosomes
Why do scientists breed plants to be polyploid?
because they will produce abnormally large flowers and fruit
Human-Inherited Disorders
Can either be caused by a gene or chromosome mutation
Human-Inherited Disorders
Can either be caused by a gene or chromosome mutation
Gene mutations are not
Chromosomal mutations can be assessed by
visible under a microscope
doing karyotypes
Karyotypes are usually carried out on
cells from a developing fetus to scan for chromosomal abnormalities such as trisomy-21
Chromosomal aberrations (abnormalities) include:
deletion, inversion, translocation, polyploidy, nondisjunction
Deletion
fragment lacking a centromere is lost during cell division
Inversion
a chromosomal fragment reattaches to its original chromosome but in reverse orientation
Translocation
fragment of a chromosome becomes attached to a nonhomologous chromosome
Polyploidy
cell or organisms has extra sets of chromosomes
Nondisjunction
homologous chromosomes fail to separate during meiosis
