ch 11 intro mendelian genetics sp11
TRANSCRIPT
Mendelian Genetics
Chapter 11
Gregor Mendel – “Father of Genetics”, first Geneticist, one of the most important scientists in history
Austrian Monk (1822-1884
Worked with ordinary garden peas
Used science and math skills learned from University.
Genetics – the scientific study of heredity
Mendel’s Peas – background information
• The anther (♂, male) produces sperm• The ovary (♀, female) produces eggs• Sperm + Eggs: FERTILIZATION• The peas Mendel started with were TRUE-
BREEDING.
Gametes
More Peas
True-breeding: parents always have offspring identical to themselves when allowed to self-pollinate. (Is self-pollination sexual or asexual reproduction?)
For example: Plants with white flowers produce only offspring with white flowers.
Important terms
Contrast with true-breeding
Trait = specific observable characteristic
Mendel produced hybrids
Hybrid The offspring produced
by parents with differing forms of a trait.
Some genetics notation Parent generation = P F1 = first generation of
offspring from a cross, (F2 = second generation, etc.)
Mendel’s crosses…results
Seed Shape
Flower Position
Seed CoatColor
Seed Color
Pod Color
Plant Height
PodShape
Round
Wrinkled
Round
Yellow
Green
Gray
White
Smooth
Constricted
Green
Yellow
Axial
Terminal
Tall
Short
Yellow Gray Smooth Green Axial Tall
Mendel’s crosses…results
Seed Shape
Flower Position
Seed CoatColor
Seed Color
Pod Color
Plant Height
PodShape
Round
Wrinkled
Round
Yellow
Green
Gray
White
Smooth
Constricted
Green
Yellow
Axial
Terminal
Tall
Short
Yellow Gray Smooth Green Axial Tall
Genes Mendel eventually concluded that traits are passed from one
generation to the next.
Today, we know that… Traits are determined by GENES
Chemical factors that determine a trait. Consist of DNA sequences
Alleles - Different forms of a gene Ex. Eye color is a trait, you could have 2 blue eye color
alleles, you could have 2 brown eye color alleles, you could have one of each.
You get one allele from each parent for each trait.
Mendel did not know anything about genes, DNA, chromosomes, meiosis, etc.
Dominant and Recessive Genes Principle of Dominance: Some alleles are
dominant and others are recessive.
Dominant: An allele that is expressed (visible) whether it is the only version of the gene present or when in combination with a recessive allele.
Recessive: An allele ONLY expressed when dominant allele is NOT present
Symbols to represent Dominant Genes If the symbol “T” represents the dominant
allele (or form of the gene), then… it doesn’t matter whether both of the alleles
an individual has are dominant (TT) or… there is just one copy of the dominant allele
(Tt). Either way, the individual looks the same (it
has the dominant trait).
Symbols to represent Recessive Genes If the symbol “t” represents the recessive
allele (or form of the gene), then… both of the alleles an individual has must be
recessive (tt) to have the recessive trait. The presence of one dominant allele (T)
hides the recessive allele (t) when there is one of each (Tt).
Important vocabulary
Homozygous two identical alleles for a trait. (ex. SS or ss) TRUE-BREEDING for a trait
Heterozygous two DIFFERENT alleles for a trait (ex. Ss) HYBRID for a trait
Phenotype physical characteristics, appearance
Genotype genetic make up, the information in DNA
End part one
Mendelian Genetics
Chapter 11
Segregation
SEGREGATION Separation of alleles during gamete formation
(meiosis) Gamete
Sex cells carry a single copy of each gene (ex. The allele
for short OR the allele for tall, not both)
Genetics and Probability
Probability likelihood that a particular event will occur. For example: What is the probability of rain
tomorrow? How likely is it that a flipped penny will land
heads-up? Tails-up? Probability is used to predict the outcomes of
genetic crosses.
Steps to a setting up a Punnett Square 1. Identify parental genotypes for the cross
(ex. TT, tt, Tt)2. Draw the Punnett Square--a grid with each
set of parental alleles on its own side.3. Separate the alleles (why?)4. Match alleles from rows and columns to
make offspring genotypes.5. Identify the kinds (and their numbers) of
offspring you have.
Gametes (sperm or eggs) are haploid (one set of chromosomes)
Steps to a setting up a Punnett Square: An Example 1. Both parents are heterozygous for plant
height, with tall being dominant to short.
2. Describe their offspring.
Probability
Look at the cross we just did. How many offspring have two alleles for
tallness (TT)? Two for shortness (tt)? Both (Tt)?
Probabilities ¼ or 25% have two alleles for tallness. ¼ or 25% have two alleles for shortness. ½ or 50% have one allele for tallness and
one for shortness. We write this in terms of a ratio 1TT: 2Tt: 1tt This ratio of allele combination types is called
the genotypic ratio.
Probabilities
How many plants will be tall and how many will be short?
We also write this in terms of a ratio (3 tall: 1 short).
This ratio of physical types is called the phenotypic ratio.
3 tall (75%), 1 short (25%)expressed as a ratio, 3:1 Why?
1 homozygous for tallness (TT) and the 2 heterozygous plants (Tt) You lump these together because you cannot tell
any of these offspring apart for this trait—TT offspring look exactly like Tt offspring.
1 homozygous recessive (tt)
This is exactly what Gregor Mendel did in his pea experiments.
A brief explanation… All of the reproduction we will be discussing
regarding Mendel’s garden peas will be sexual reproduction (a new organism formed by the joining together of a male and a female gamete).
However, pay close attention to who the parents are — sometimes there is a cross between two different parents (usually with contrasting forms of some trait, like flower color or height). Other times, the peas are allowed to do what comes naturally to peas—self-pollination (the same plant fertilizes itself).
Sexual Reproduction in Plants
Female (♀) sexual organs
Male (♂) sexual organs
Cross-pollination – either by wind, insect or geneticist Pollen
(containing male gametes)
Self-pollination
Convenient, but boring
Mendel’s first crosses The parent
generation (P) is a combination of two different pure-breeding types
The first generation (F1) produced only TALL plants
The second generation (F2 ) produced tall and short plants
Mendel’s Monohybrid cross
Tall allele/Tall allele
TT short allele/short allele
tt
Predictions
Mendel found that the approximately three dominant to one recessive ratios showed up consistently.
Probabilities predict average outcomes for a LARGE number of events, not exact outcomes.
Flip a coin twice Heads once, tails once Heads twice Tails twice
Flip a coin many times, likely to get very close to a 50:50 ratio
Predicting Genetics
Genetics is similar Larger numbers result in closer to expected
values. This explains why humans don’t always have
the same number of male children and female children despite the fact that the odds of any one child’s gender is 50% female, 50% male.
Mendel’s Principle Independent Assortment
Genes for different traits segregate independently during gamete formation
Accounts for much of the genetic variation in living things Usually, all combinations of traits are possible, ex. Tall round seeds, purple flowers
Short round seeds, white flowers
Short wrinkled seeds purple flowers
Tall wrinkled seeds white flowers
Etc.
Summary of Mendel’s Principles
Genes determine inheritance. Genes are passed from parent to offspring.
Some genes may be dominant and others recessive
Adults (sexually reproducing) have two copies of each gene—one from each parent. They segregate during gamete formation.
Alleles for different traits assort independently (Independent Assortment)