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What is Genetics?Genetics is the scientific study of
heredity
Genomes
A genome is the full set of genetic information that an organism carries in its DNA.
The analysis of any genome starts with chromosomes.
A human karyotype
Karyotypes
A karyotype shows the complete diploid set of chromosomes grouped together in pairs, arranged in order of decreasing size.
Autosomal Chromosomes
The other 22 pairs of non-sex chromosomes are called autosomal or autosomes.
Male = 46, XY
Female = 46, XX
Transmission of Human Traits
Many human traits follow a pattern of simple dominance.
For example, the gene MC1R helps determine skin and hair color.
Some of MC1R’s recessive alleles produce red hair.
An individual with red hair usually has two of these recessive alleles.
Transmission of Human Traits
The X and Y chromosomes determine sex.
The genes located on them show sex linkage.
X-Chromosome Inactivation
Most genes in one of the X chromosomes are switched off, forming a region known as a Barr body.
Female calico cats are
tri-colored.
Male cats can have spots
of only one color.
The color of spots on
their fur is controlled by a
gene on the X
chromosome.
Gregor Mendel monastery priest (1st important studies of inheritance)
Heredity – the passing on of characteristics from
parents to offspring
Genetics is the branch of biology that studies the
patterns of inheritance and variations in organisms
Mendel was the first to succeed in predicting how traits
would transfer from one generation to the next.
Mendel’s PEAs!!! https://www.youtube.com/watch?v=Mehz7tCxjSE
Rule of Dominance: if you have 2 different alleles
and 1 is dominant, that is what trait will be
expressed.
• If the F1 generation plants (second generation) had a tall allele from one parent and a short allele from the other, they all appeared tall.
• Therefore, one trait was dominant over the other trait (recessive).
• Plants with two alleles for tallness (TT) were tall.
• Plants with two alleles for shortness were short (tt)
• Plants with one allele for tallness and one for short (Tt) were tall
Law of Segregation: only 1 copy of a
gene is found in a gamete• If an organism has two different alleles (genes) for a trait,
that organism can make two different types of gametes.
– Tt plant can produce T gametes and t gametes
• Fertilization from a Tt + Tt cross will result in random
pairs of the available gametes
(four possible combinations)
– T from male + T from female = TT
– T from male + t from female = Tt
– t from male + T from female = Tt
– t from male + t from female = tt
YES – this is related to Punnett Squares!!!
Sex Chromosomes
Two of the chromosomes in the human genome are sex chromosomes.
Females: two X chromosomes
Males: one X chromosome; one Y chromosome
Dihybrid Punnett Square Put one of each type of possible gamete combination from one
parent on top of the square
Put one of each type of possible gamete combination from the other parent on the side of the square
Fill each box with the gamete of that box’s row and column
The possible offspring combinations can be seen
The ratio of offspring phenotypes after a heterozygous dihybrid cross is 9:3:3:1 In this case we see 9 round yellow,
3 round green, 3 wrinkled yellow and 1 wrinkled green
What is a Trait?A trait is a specific characteristic that varies
from one individual to another.
Examples: Brown hair, blue eyes, tall, curly
What is an Allele?Alleles are the different
possibilities for a given trait.
Every trait has at least two alleles (one from the mother and one from the father)
What are Genes?Genes are the
sequence of DNA that codes for a protein and thus determines a trait.
Dominant vs. Recessive
Dominant - Masks the other trait; the trait that shows if present
Represented by a capital letter
Recessive – An organism with a recessive allele for a particular trait will only exhibit that trait when the dominant allele is not present; Will only show if both alleles are present
Represented by a lower case letter
R
r
Dominant & Recessive Practice
TT - Represent offspring with straight hair
Tt - Represent offspring with straight hair
tt - Represents offspring with curly hair
T – straight hair
t - curly hair
Genotype vs. Phenotype Genotype – The genetic makeup of an organism;
The gene (or allele) combination an organism has.
Example: Tt, ss, GG, Ww
Phenotype – The physical characteristics of an organism; The way an organism looks
Example: Curly hair, straight hair, blue eyes, tall, green
Homozygous vs. Heterozygous
Homozygous – Term used to refer to an organism that has two identical alleles for a particular trait (TT or tt)
Think homo=same
Heterozygous - Term used to refer to an organism that has two different alleles for the same trait (Tt)
RR
Rr
rr
Punnett SquaresPunnett Square – Diagram showing the
gene combinations that might result from a genetic cross
Used to calculate the probability of inheriting a particular trait
Probability – The chancethat a given event will occur
Punnett Square
Parent
Parent Offspring
How to Complete a Punnett Square
Y-Yellow
y-white
Genotype:
1:2:1
(YY:Yy:yy)
Phenotype:
3 Yellow
1 White
You Try It Now! Give the genotype and phenotype for the following
cross: TT x tt (T = Tall and t = Short)
TT x ttStep One: Set Up Punnett Square (put one parent on the top
and the other along the side)
T Tt
t
TT x ttStep Two: Complete the Punnett Square
T Tt
t
Tt Tt
Tt Tt
TT x ttStep Three: Write the genotype and phenotype
T Tt
t
Tt Tt
Tt Tt
Genotype:
4 - Tt
Phenotype:
100% Tall
Remember: Each box is 25%
You Try It Now! Give the genotype and phenotype for the following
cross: Tt x tt
Tt x ttStep One: Set Up Punnett Square (put one parent on the top
and the other along the side)
T tt
t
Tt x ttStep Two: Complete the Punnett Square
T tt
t
Tt tt
Tt tt
Tt x ttStep Two: Complete the Punnett Square
T tt
t
Tt tt
Tt tt
Genotype:
Tt - 2 (50%)
tt - 2 (50%)
Phenotype:
50% Tall
50% Short
Remember: Each box is 25%
Incomplete Dominance Incomplete Dominance - Situation in
which one allele is not completely dominant over another.
Example – Red and white flowers are crossed and pink flowers are produced.
Codominance Codominance - Situation in which both
alleles of a gene contribute to the phenotype of the organism.
Example – A solid white cow is crossed with a solid brown cow and the resulting offspring are spotted brown and white (called roan).
+
Blood Type Human blood types demonstrate multiple alleles (more
than two alleles of the gene) A, B, O
Human blood types also demonstrate codominance –where heterozygous alleles can be expressed equally A and B are codominant
O is recessive
These alleles are written as IA, IB, and i IAIA or IAi will have type A blood
IBIB or IBi will have type B blood
IAIB will have type AB blood
ii will have type O blood
Blood Type Crosses
Type O mother and Type AB father
IA
i IAi
50% blood type A
50% blood type B
100% type AB
i IBi
IBi
IAi
IB
IA IAIB
IA
Homozygous Type A mother and
Homozygous Type B father
IB IB
IAIB
IAIBIAIB
Sex Chromosomes
More than 1200 genes are found on the human X chromosome.
The human Y chromosome contains only about 140 genes.
Sex-Linked Traits• Traits controlled by genes carried on the X or Y
chromosomes are called sex-linked traits
• Most of these types of traits are carried on the X chromosome
• The alleles for different forms of the sex-linked traits are shown as superscripts on the X
– XRXR XRXr XrXr
– XRY XrY
• Because the Y does not carry an allele, a male could not be heterozygous for a sex-linked trait
Sex-Linked Crosses
Colorblind mother and Normal father
XN Y
Xn XNXn
Normal daughters (carriers)
Colorblind sons
50% Normal daughters and sons
50% colorblind daughters and sons
Colorblindness is a recessive sex-linked trait. Use XN for the normal allele and
Xn for the colorblind alleleHeterozygous Normal mother
and Colorblind father
Xn XnY
XnY
XNXn
Xn Y
XN XNXn
Xn XnY
XNY
XnXn
Examples of Multiple Alleles
1. Blood Type – 3 alleles exist (IA, IB, and i), which results in four different possible blood types
2. Hair Color – Too many alleles exist to count
There are over 20 different shades of hair color.
Multiple Alleles There Are Always Multiple Alleles!
Genetic inheritance is often presented with straightforward examples involving only two alleles with clear-cut dominance. This makes inheritance patterns easy to see.
But very few traits actually only have two alleles with clear-cut dominance. As we learn more about genetics, we have found that there are often hundreds of alleles for any particular gene.
We probably know this already - as we look around at other people, we see infinite variation.
Polygenic Trait Polygenic Trait - Trait
controlled by two or more genes.
Polygenic traits often show a wide range of phenotypes.
Example: The wide range of skin color in humans comes about partly because more than four different genes probably control this trait.
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