Transcript Mendelian genetics

MENDELIAN GENETICS
LEARNING OBJECTIVES
SWBAT: Describe the connection between the
process of meiosis and the passage of traits
from parent to offspring
SWBAT: Apply mathematics to determine
Mendelian patterns of inheritance.
SWBAT: Explain how the inheritance patterns
of many traits cannot be accounted for by
Mendelian genetics.
What do you already know about
Gregor Mendel and/or Mendelian
genetics?
GREGOR MENDEL
 Prevailing theory of inheritance 1800’s = blending
 Problem: over time and many generations, a very
uniform population would be produced
 Mendel introduced the idea of a “particulate”
method of inheritance.
 Parents pass on discrete units of heritable material
that can be shuffled and recombined in a number of
ways.
 All this BEFORE scientists knew anything about
chromosomes and the genes they were made of!
TERMS TO KNOW
True- breeding plants - plants that over many
generations of self-pollinating, produce only the
same variety as the parent plant
Hybridization - mating or crossing of 2 truebreeding varieties
P generation - the true-breeding parents
F 1 generation - the first generation hybrid offspring
F 2 generation - F 1 self-pollination offspring
generation (F 1 X F 1 )
WHY PEA PLANTS?
Many varieties with easily measurable
and distinct characters.
Mating could be strictly controlled
assuring parentage
Only chose to track characters with 2
distinct alternative forms
 Mendel’s analysis of 1000’s of genetic crosses to F 2
helped him to develop two important principles of
heredity.
The Law of Segregation
The Law of Independent Assortment
THE LAW OF SEGREGATION
 By analyzing many F 1 crosses for seven
characteristics in pea plants, he noted 2 things
 All F 1 generation plants displayed the same
character. He called this the dominant trait
 In F 2, the offspring displayed the trait in a 3:1 ratio,
dominant: recessive.
This meant that the recessive trait did not disappear,
but was hidden
 He developed a model to explain what he observed.
F I G U R E 11 . 3 - 3
Experiment
P Generation
(true-breeding
parents)
Purple flowers
White flowers
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
F2 Generation
705 purple-flowered 224 white-flowered
plants
plants
THE LAW OF SEGREGATION
Concepts included in his model and now applied to current
knowledge of genes:
1. Alternative versions of genes account for variations in
inherited characters. Now called alleles
THE LAW OF SEGREGATION
2. For each character, an organism inherits 2 copies
of a gene, one from each parent.
Remember- he knew nothing about DNA or
chromosomes!
THE LAW OF SEGREGATION
3. If the two alleles at a locus dif fer, the dominant allele
determines the organism’s appearance ( phenotype); the
other, the recessive allele, has no noticeable ef fect.
THE LAW OF SEGREGATION
The two alleles for a heritable character
segregate (separate from each other) during
gamete formation and end up in different
gametes.
When does this happen? During which phase
of meiosis?
Or, put another way . . . When does the cell
become haploid?
Homologous chromosomes segregate in
Anaphase 1
Punnett squares provide a visual
representation of the law of
segregation and can help predict the
outcomes of fertilization .
MORE TERMS TO KNOW
Homozygous= having two identical alleles for
a given gene
Heterozygous= having two different alleles for
a given gene
Genotype= the genetic makeup, or set of
alleles, of at organism
Phenotype = the observable physical and
physiological traits of an organism, which are
determined by its genetic makeup
TEST CROSS
Used to determine the genotype of
an organism with the dominant
phenotype
THE LAW OF INDEPENDENT ASSORTMENT
The second important principle of heredity
developed by Mendel
Looked at true breeds that differed in 2
characters
Wanted to know if the 2 characters were
transmitted together
Dihybrid cross
F I G U R E 11 . 8
Experiment
YYRR
P Generation
yyrr
Gametes YR
yr
F1 Generation
YyRr
Hypothesis of
dependent assortment
Predictions
Hypothesis of
independent assortment
Sperm
or
Predicted
offspring in
F2 generation
¼ YR ¼ Yr ¼ yR ¼ yr
Sperm
½ YR ½ yr
¼ YR
½ YR
Eggs
½ yr
YYRR YyRr
YyRr
¾
yyrr
¼ Yr
Eggs
¼ yR
YYRR YYRr
YyRR YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR YyRr
yyRR
yyRr
YyRr
yyRr
yyrr
¼
Phenotypic ratio 3:1
¼ yr
9
16
3
16
Yyrr
3
16
1
16
Phenotypic ratio 9:3:3:1
Results
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
THE LAW OF INDEPENDENT ASSORTMENT
Two or more genes assort independently.
That is, each pair of alleles segregates
independently of each other pair during
gamete formation.
Note: This applies only to allele pairs on
different chromosomes, or very far apart
on the same chromosome
PROBABILIT Y LAWS
Rule of multiplication:
The probability that 2 or more
independent events will occur together in
a specific combination – multiply the
probability of each
Some inheritance patterns are more
complex than simple Mendelian genetics
Complete dominance
The Mendelian idea of dominant and recessive
alleles. Homozygous dominant and
heterozygous look the same
CodominanceBoth alleles “show” (roan horses) or there is
more than one dominant
Incomplete dominance
The heterozygous shows a blending of the two
traits
Multiple alleles
Genes that have more that 2 possible
alleles
Again like ABO
Polygenetic inheritance
2 or more genes have an additive affect on a
single character
Skin color
Height
WHAT IS THE SPECIFIC INHERITANCE
PATTERN OF THE FOLLOWING DISEASES?
Cystic fibrosis
Tay-Sachs
Sickle cell
Huntington’s
LEARNING OBJECTIVES
SWBAT: Describe the connection between the
process of meiosis and the passage of traits
from parent to offspring
SWBAT: Apply mathematics to determine
Mendelian patterns of inheritance.
SWBAT: Explain how the inheritance patterns
of many traits cannot be accounted for by
Mendelian genetics.
HOMEWORK
FRQ
Concept 11.1 text p. 212 (optional
concept 11.2 p. 214)-TURN IN
Inheritance patterns of diseases
TOMORROW
Sex linked inheritance
Recombination and linkage
Biotech basics
PCR
Micropipettes
Gel electrophoresis
The “taster gene”
LEARNING OBJECTIVES
SWBAT: Describe the connection between the
process of meiosis and the passage of traits
from parent to offspring
SWBAT: Apply mathematics to determine
Mendelian patterns of inheritance.
SWBAT: Explain how the inheritance patterns
of many traits cannot be accounted for by
Mendelian genetics.