Download Chapter 11

GENETIC TRAITS…WHICH DO YOU HAVE?
 Widows peak vs. straight hairline
 Free earlobes vs. attached earlobes
 Dimples vs. no dimples
 Cleft chin vs. no cleft chin
 Straight thumb vs. curved thumb
 Mid-digit hair vs. no mid-digit hair
 Longer 2nd toe vs. shorter 2nd toe (compared to big toe)
 Can roll tongue vs. can’t roll tongue
CHAPTER 11
Introduction to Genetics
11-1 THE WORK OF GREGOR MENDEL
 Heredity: transmitting genetic information from
parent to offspring
 Genetics: the study of heredity
 Gene: a piece of DNA that codes for a trait
 Alleles: a different form of a gene
 Trait: specific characteristic that varies between
individuals
 Gametes: specialized cells involved in sexual
reproduction (eggs and sperm)
MENDEL AND HIS PEAS
A priest with a garden…
Studied pea plants
Male part = stamen
Female part = pistil
Fertilization: joining of male and female parts
Form a new cell (seed)
PEA PLANTS = TRUE BREEDING
True-breeding: pass on ALL genetic traits to
offspring
Self-pollinating- use male and female parts from
same plant
Breed offspring identical to parent
CROSS-BREEDING
Using male and female parts from different plants
Mendel attempted this with pea plants
Aka: cross pollination
SO WHAT DID MENDEL DO?
Identified 7 traits
each with 2 contrasting characteristics
Crossed those characteristics
Resulted in hybrid plants
P generation (parents)
F1 generation (offspring)
WHAT DID HE FIND?
Offspring only showed the traits of one parent
2 conclusions:
1. Inheritance is determined by factors passed
from parent to offspring (genes and alleles)
2. Principle of dominance
some alleles are dominant others
recessive
WHERE DID THE RECESSIVE TRAIT GO?
 Next, Mendel allowed self pollination
to occur again
 Results of F2 generation (offspring’s offspring)…
 ¾ of the plants = showed dominant
trait
 ¼ of plants = showed recessive trait
 The recessive trait was still
there…just “hiding”
SEGREGATION
Separation of alleles during gamete formation
Occurs so gametes only carry one type of allele
Why we saw only one type of the trait
Mom
II
I
gametes
Dad
II
I
11-2 PROBABILITY AND PUNNETT
SQUARES
 Mendel repeated experiments, got same
results
 Probability: likelihood that an event will
occur
 Ex: coin flip (1 in 2 odds)
 Each turn is independent of any previous or
future turns
 Principles of probability can be used to
predict outcomes of genetic crosses
PUNNETT SQUARES
Used to predict/compare the genetic variations
that will result from a cross
Alleles:
capital letter = dominant
lowercase letter = recessive
PUNNETT SQUARES
(Tt)
Parent’s alleles
The boxes in the
square contains
every possible
combination of
alleles
(Tt)
PUNNETT SQUARES
 TT or tt = homozygous (2 identical alleles)
 Tt = heterozygous (2 different alleles)
 TT or Tt = dominant allele will show
 tt = recessive allele will show
 ¾ = dominant allele shown
 ¼ = recessive allele shown
 *3:1 ratio for dominant trait
GENOTYPE VS. PHENOTYPE
Genotype: genetic makeup
What the alleles are (TT, Tt or tt)
Phenotype: physical characteristics
What you see (tall or short)
TT or
Tt
tt
PROBABILITY AND PREDICTIONS
REMEMBER: probability only predicts averages
NOT precise outcomes
Larger numbers of trials means outcomes closer to
the probability
PUNNETT SQUARE EXAMPLE QUESTIONS
 In seals, whisker length is determined by two alleles. “W”
codes for long whiskers and is dominant over “w” which
codes for short whiskers. Draw a Punnett square to show
a cross between a homozygous long whiskered dad and a
heterozygous long whiskered mom.
 What percentage of their offspring would have long
whiskers? Short whiskers?
 What percentage of their offspring would be homozygous?
What percentage would be heterozygous?
PUNNETT SQUARE EXAMPLE ANSWERS
 What percentage of their offspring would have long
whiskers?
 100%
 What percentage would have short whiskers?
 0%
 What percentage of their offspring would be homozygous?
 50%
 What percentage would be heterozygous?
 50%
11-3 EXPLORING MENDELIAN GENETICS
Mendel’s next question: do alleles separate
independently?
Tried crossing 2 traits at once
Found that round (R) peas and yellow (Y) peas
were dominant traits
Would they stay together or separate?
LAW OF INDEPENDENT ASSORTMENT
Genes for different traits will segregate
independently of each other during gamete
formation
Accounts for all genetic variation!
SUMMARY OF MENDEL’S PRINCIPLES
 Inheritance of traits is determined by genes (alleles) passed
from parent to offspring
 When 2 or more alleles exist, some will be dominant and
some will be recessive
 In sexually reproducing organisms, each adult has 2 copies
of each gene which separate when gametes are formed
 Alleles for different traits usually separate independently
EXCEPTIONS TO THE RULES
Genetics is very complicated
Some alleles are not simply dominant/recessive
Some traits controlled by multiple genes
INCOMPLETE DOMINANCE
One allele is not completely dominant over
another
A “mixing” occurs
Ex: Mirabilis plant- crossing red flowers with white
flowers gives you pink flowers
X
CODOMINANCE
2 dominant alleles (both will contribute to the
phenotype)
Both alleles are present and can be “seen”
Ex: erminette chicken- has both black and white
feathers
MULTIPLE ALLELES
3 or more alleles available
Gives more options for the combination of alleles
Ex: blood types
POLYGENIC TRAITS
Traits controlled by 2 or more genes
Ex: skin color
LOTS of variation
APPLYING MENDEL’S PRINCIPLES
Thomas Hunt Morgan
Used fruit flies
All principles held true
GENETICS AND THE ENVIRONMENT
Genes act as the “blue print” plan for living things
Environment will influence how the plan works
Ex: sunflower
 Height and color determined by genes
 Influenced by water, light, temperature, soil, etc.)
11-4 MEIOSIS
Mendel’s principles require 2 things:
 1. Organisms inherit single copy of genes from each parent
 2. Therefore, when gametes are formed, those copies must
separate
But…how?
CHROMOSOMES
Remember: genes are found on chromosomes
Half come from mom and half come from dad
Each chromosome has a “corresponding
chromosome”
(1 from mom and 1 from dad)
Known as homologous chromosomes
CHROMOSOMES
Diploid- cells that contain both sets of
homologous chromosomes
Haploid- cells that contain only a single set of
homologous chromosomes
Gametes!
MEIOSIS
Meiosis: the process where chromosome # is cut
in half by separating homologous chromosomes in
diploid cells (makes gametes)
Turns 1 diploid cell into 4 haploid cells
Involves meiosis I and meiosis II
MEIOSIS I
Must go thru interphase first to replicate
chromosomes
Prophase I- chromosomes pair with homologous
chromosomes (these are called tetrads)
Crossing over- exchange parts of chromatids
 Very important for genetic variation
 Get new allele combinations
MEIOSIS 1 (CONTINUED)
Metaphase I- line up across middle, attach to
spindle
Anaphase I- spindle pulls homologous
chromosomes apart
Telophase I- two new nuclear membranes form
Cytokinesis- two new diploid cells are formed
IMPORTANT!
After Meiosis 1, the 2 new diploid cells have
“shuffled” sets of chromosomes
 From “crossing over”
Different from each other
Different from original cell
Genetic variation
MEIOSIS II
Prophase II- sister chromatids condense
Metaphase II- sister chromatids line up
Anaphase II- sister chromatids separate
Telophase II and cytokinesis = 4 new haploid cells
GAMETE FORMATION
Males = sperm
Females = eggs
 usually, only 1 of the 4 produced are used in
reproduction (actually become an egg)
MITOSIS VS. MEIOSIS
Mitosis
 End with 2 genetically identical diploid cells
 Role: growth and replacement of cells
 Asexual reproduction
Meiosis
 End with 4 genetically different haploid cells
 Role: make gametes
11-5 LINKAGE AND GENE MAPS
Linkage: if genes are close to each other on a
chromosome, they may be inherited “linked”
together
chromosomes follow the Law of Independent
Assortment…NOT individual genes
GENE MAPS
Gene maps: shows the relative locations of genes
on a chromosome
How can we tell this?
 The closer together genes are, the more likely they are
to be “linked”
Human Genome Project
ANNEMARIE PARISI…
Will you go to prom with me?
-EJ