Mendel’s Laws of Heredity
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Transcript of Mendel’s Laws of Heredity
Mendel’s Laws of Heredity
Section 10.1 p. 253 - 262Chapter 10 Mendel and
Meiosis
• Mendel chose which pea plants he would allow to fertilize or pollinate one another and then kept records of the offspring produced.
Before we can go any further…
• There are a bunch of vocab words we need to start using.– Alleles– Dominant– Recessive– Homozygous– Heterozygous
Mendel’s ~ The Rule of Unit Factors
• Each organism has two factors that control each of its traits.
• These factors are now known as genes.
• Genes come in different forms.
• Alternative forms of a gene for each variation of a trait is an allele.
alleles
• We get two copies of each allele or directions for a trait.
• One copy comes from each parent.
• We may get a matching set or they may be different with the dominant one hiding or masking the recessive version.
dominant
• Observed trait of an organism that masks the recessive form of a trait.
• If present, this is the trait we see.
• “Dominant” is not always “good”.
recessive
• Trait of an organism that can be masked by the dominant form of a trait.
• You need two copies, one from each parent, for a recessive trait to be expressed.
• Recessive traits are not always “bad”.
Mendel’s ~ The Rule of Dominance
• Even though an organism carries two alleles for the same trait, only one trait is observed.
• The dominant trait will be expressed.
Homozygous
• Having two of the same alleles for a gene
• When there are two identical alleles for a trait.
Heterozygous
• Having two different alleles for the same gene.
• Back to the peas, each pea has two height genes.
• T = tall, t = short
• TT and tt are homozygous.
• Tt is heterozygous.
Phenotype
• Outward appearance of an organism, regardless of its genes.
• An organism's traits (like green vs. yellow seeds, or pea plant height)
• What you look like!
Genotype
• Combination of genes in an organism.
• Can not tell from outward appearance
• Set of directions on the inside.
Genotype
• If it is a recessive trait, the genotype is obvious.
• Dominant traits are expressed whether the organism is heterozygous (2 different alleles) or homozygous (2 of same alleles) so you do not know for sure just from looking.
hybrid
• Offspring formed by parents having different forms of a trait.
• Mendel’s crossbreed strains were made breeding know true-breeding plants to different true-breeding plants.
• Working with his peas, Mendel used 355 cross-bred strains and 12,980 resultant hybrids.
True-breeding or homozygous
• True-breeding means that they only have one version of a trait to pass on or homozygous.
• Sometimes we refer to homozygous organisms as being a “pure bred”.
• If a pea plant came from parents that were tall, it is tall and all of its offspring are tall when crossed with other pea plants that are “true-breeding”, we can be fairly certain that it is homozygous.
Poodles
Wolves
• What would their puppies look like?
• In 1976 a person named C. Schleifenbaum lead a study to find out.
poodle-wolf hybrids
• They bred wolves to poodles.
• Then looked at the fur marks and coat color in wolves and poodle-wolf hybrids.
• Here are some pictures of their results.
1st generation ~ poodle-wolf hybrid
Notice how the coats range from wolf-like to poodle-like
2nd generation ~ poodle-wolf hybrid
Monohybrid cross
• How can we explain the poodle-wolf hybrid results?
• Poodles are “true-breeding” for one version of traits.
• Wolves have a different version of these same traits.
Monohybrid cross
• In a monohybrid cross, the parents selected are true-breeding for the trait we will be studying.
• The parents are known as the P1
generation.
• Two P1 parents will be breed or crossed and the resulting offspring are the 1st or F1 generation.
1st generation
• First generation is the F1 generation
• “F” stands for filial ~ son or daughter
• In the F1 generation, all of the offspring should be similar, showing all the dominant traits that were passed on by the P1 generation.
1st generation
• All of the poodle-wolf hybrids looked the same.
Mendel’s ~ The Law of Segregation
• Each plant has two different alleles, it can produce two different types of gametes.
• During fertilization, male and female gametes randomly pair to produce four combinations of alleles.
The Law of Segregation
Homozygous P1 =
All offspring will have the same genotype
They will all have the same phenotypes & look like the dominant trait.
Heterozygous P1 =
The offspring will have a 1:2:1 ration in genotypes
3:1 ration in phenotypes
Monohybrid Crosses
Homozygous crosses
• Mendel choose plants that were breeding true.
• His peas must have been homozygous…
• Only able to provide one type of allele or variety of that particular trait.
Homozygous crosses
• Mendel’s first pea plant crosses were between tall pea plants and short pea plants.
• One parent has two tall alleles and the other parent has two short alleles.
• They are both homozygous for their trait.
How we represent the parents & offspring in a
cross• A letter is selected to represent a
trait.
• The dominant version is shown with the capital.
• The recessive version is shown with the lower case .
How we represent the parents in a cross
• With the height of pea plants a capital T is used to represent the tall version
• A lower case t is used to represent the short version
Genotypes in the cross
• The tall pea parents will be = – TT, it can only give T alleles
• The short pea parents will be =– tt, it can only give t alleles
Punnett Squares
• A punnett square is a way to predict the possible outcome of a breeding cross
• Each box represents a reproductive opportunity or offspring.
Short parent’s alleles
t
tTall parent’s alleles
T
T
Punnett Squares
• The offspring gets one allele from each parent so they end up with two copies of alleles for each trait.
• Divide each parent’s two alleles so that each box has one letter next to or above it
Short parent’s alleles
t
tTall parent’s alleles
T
T
Monohybrid cross - Homozygous
• If one parent is homozygous dominant for their trait and the other is homozygous recessive, the offspring will all have the same phenotype and genotype.
Short parent’s alleles
t
tTall parent’s alleles
T
T
• The offspring’s genotype (what the alleles say) will be =
• Tt• The offspring’s
phenotype (what we see) will be =
• The plants will be tall.
• The ________ is the dominant trait.
Short parent’s alleles
t
tTall parent’s alleles
T
T
Monohybrid cross - Homozygous
T
Monohybrid cross
Both Homozygous/
1 –Recessive
1 -Dominant
t
T t
t t
t
t
T
T
T
TGenotype
Genotype
Genotype
Genotype
Phenotype =
Phenotype =
Phenotype =
Phenotype =
Dominant
DominantDominant
Dominant
2nd generation
• Second generation is the F2 generation
• The second generation all has two
different copies (heterozygous) for each trait.
• Their offspring will show more variation because the alleles will regroup in many ways.
Monohybrid cross - Heterozygous
• If two organisms are selected out of the F1 generation to be parents for the next generation, they will each be heterozygous.
• Sticking with our offspring of a tall pea plant & and a short pea plant, this group will have a genotype of Tt.
• They will have a phenotype of tall.
Monohybrid cross - Heterozygous
• Remember, the offspring get one allele from each parent so they end up with two copies of alleles for each trait.
• Both parents are heterozygous so they have a genotype of Tt.
Heterozygou
s parent’s alleles
T
tHeterozygou
s parent’s alleles
T
t
Monohybrid cross - Heterozygous
• How many of the offspring will be tall?
• How many of the offspring will be short?
Heterozygou
s parent’s alleles
T
tHeterozygou
s parent’s alleles
T
t
Monohybrid cross - Heterozygous
• What are the possible genotypes for the offspring?
• What is the ratio of tall (dominant) to short (recessive) plants in the F2 generation?
Heterozygou
s parent’s alleles
T
tHeterozygou
s parent’s alleles
T
t
T
Monohybrid cross
Both Heterozygous
T
t T
T t
t
t
t
T
t
TGenotype
Genotype
Genotype
Genotype
Phenotype =
Phenotype =
Phenotype =
Phenotype =
Dominant
Recessive
Dominant
Dominant
2nd generation
• The second generation of poodle-wolf hybrids looks very different from each other!
Crossing Homozygous Pea Plants
Crossing Heterozygous Pea Plants