Transcript Introduction to Genetics

Introduction to Genetics
Sexual Reproduction:
– Zygote is a product of both parents
– The chromosomes of each gamete (egg and
sperm) bring hereditary material from each
parent
– Offspring is both similar and different from
both parents
Phenotypes and Genotypes
• Phenotype: the way an organism looks or
appears
• Genotype: the actual gene combination
an organism has (may not be visually
apparent)
Development of Genetic
Theories
Gregor Mendel
(Mid 1800s)
• Studied pea plants.
• How were traits passed on from
one generation to the next
• Discovered basic laws of genetics
• Crossed plants that were purebred
for different traits
– Purebred: organisms that produced
organisms identical to themselves
• Studied one trait at a time
Mendel’s First Experiment
Monohybrid Cross:
– Parents differ by a single trait
– Offspring of this type of cross are called hybrids
• Ex: Mendel crossed tall and short pea plants
• Parent Generation (P1)
• 1st generation (F1):
– All tall offspring
– What happened to the
short?
• 2nd generation (F2):
– ¾ tall and ¼ short
– The short trait
reappeared!!
• He repeated his tests for 7 other traits in
pea plants and got same result.
– One trait disappears in F1 generation
– But reappears in ¼ of F2 generation
WHY???
• Bill Nye:
• Greatest Discoveries: Law of Inheritance
• http://www.youtube.com/watch?v=0vAAf4g
5iF8&safe=active
• Online Activity
• http://www.hhmi.org/coolscience/resources
/SPT--FullRecord.php?ResourceId=73
Mendel’s Rules for Inheritance
Rule of Unit “Factors”
Characteristics are determined by factors
that are passed from one generation to the
next
– Each organism has 2 “factors” for each trait
– These “factors” can exist in alternate forms
We now call these factors
genes
These genes can exist in
different forms or alleles
Organisms inherit one allele
from each parent
(on homologous chromosomes)
Mendel’s Rules for Inheritance
Rule of Dominance
• Some alleles are dominant and some recessive
• EX:
– TT and tt pea plants are crossed
– All offspring are:
• Genotypically Tt (hybrid)
• Phenotypically Tall
Genotype
Phenotype
TT
Tall
Tt
Tall
tt
Short
– Trait that phenotypically appears is dominant (Tall)
– Trait that disappears is recessive (short)
Dominance in Human Traits
Dominant
Freckles
Curly hair
Recessive
No freckles
Straight hair
Hair on middle joint of finger
No hair on middle joint of finger
Widows peak
Unattached earlobe
Curl tongue
No widows peak
Attached earlobe
Can’t curl tongue
Mendel’s Rules for Inheritance
Law of Segregation
How are different forms of a
gene distributed to offspring?
During gamete formation
(meiosis), alleles for each
gene segregate or separate
independently from each other
Each gamete only carries one
allele for each gene.
• Homozygous: Individuals that have two of
the same alleles for a trait (ex: BB or bb)
• Heterozygous: “hybrid” individuals that
have two different alleles for the same
trait. (ex: Bb)
Punnet Squares
• Illustrates the law of segregation of traits
• Shows all possible combination of gametes
• Way of finding expected proportions of traits
in offspring
– (Need a large sample size to really get
expected ratio)
• Monohybrid Cross:
• Ex: Parents differ by single trait for height
– TT x tt
– Tt x Tt
– Tt x tt
– Are the parents in each cross
homozygous/heterozygous?
– What are the phenotypes for each parent?
– Let’s cross
– What are the outcomes?
– What are the genotypes and phenotypes of
the offspring?
• Dihybrid Cross:
– Cross involving two different traits
– Ex: Round yellow seeds x wrinkled green seeds
– Round = R, Wrinkled = r
– Yellow = Y, Green = y
– Let’s try a cross! RRYY x rryy
– P1 Generation:
RRYY x rryy
– F1 Generation:
All round yellow (RrYy)
– 100% heterozygous hybrids for both traits
– Phenotype is 100% yellow round
– Round and yellow traits must be dominant!
– F2 Generation:
– Cross “dihybrids” from the first generation
• RrYy x RrYy
• Create a Punnet Square:
– What goes at top of each column and row?
– What are the possible combination of these genes
in the gametes?
– Phenotype Outcome:
Yellow Round
9
Green Round
3
9:3:3:1
Yellow Wrinkled
3
Green Wrinkled
1
• Do you get a 9:3:3:1 phenotype ratio?
Law of Independent Assortment
Alleles for different traits are inherited independently of
each other.
During gamete formation (meiosis) chromosomes are
separating independent of each other
• Ex:
– Round/wrinkled and yellow/green traits are inherited
independantly of each other
• Note:
– Only true if the genes for traits are located on different
chromosomes
Mendelian Genetics Summary
Law of Unit Factors:
– Traits have 2 factors (or alleles) that can be inherited from parents
Law of Dominance:
– One factor is dominant and the other recessive
Law of Segregation:
– During gamete formation the factors segregate randomly into eggs
and sperm and then recombine in offspring
Law of Independent Assortment:
– Different traits sort themselves independently of each other
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Phenotype (how it looks) vs. Genotype (what genes/alleles it has)
Homozygous (BB or bb) vs. Heterozygous (Bb)
Monohybrid Cross (hybrid for one trait) get 3:1 phenotype ratio
Dihybrid Cross (hybrids for 2 traits) get 9:3:3:1 phenotype ratio
Punnet Square: know how to set them up!