Mendel`s Laws of Heredity

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Transcript Mendel`s Laws of Heredity

Mendel’s Laws of Heredity
• Heredity- the passing on of characteristics
from parent to offspring
• Characteristics that are inherited are
called traits
• Genetics- the branch of biology that
studies heredity
Gregor Mendel
• In the mid-19th century, Gregor Mendel, an
Austrian monk carried out important
studies of heredity
• He was the first to predict how traits are
transferred from one generation to the
Mendel’s Pea Plants
His experiments were conducted using
pea plants because:
1. The reproduce sexually through the
formation of gametes
2. He could control fertilization, by
pollinating the plants himself
Monohybrid Crosses
• A hybrid is the offspring of parents that
have different forms of a trait, such as tall
and short heights
• Mendel’s 1st experiments are called
monohybrid crosses because he looked at
only one trait
Parental Generation
• Parental generation- P1- each parent plant
was from other plants that displayed the
same traits
• He cross-pollinated the tall plant with the
short plant
First Generation
• The offspring of the P1 cross are called
the first generation or F1 generation
• In this generation, all of the pea plants
would be tall
• In the F1 generation, it was as if the
shorter parent had never existed
Second Generation
• Mendel allowed the tall plants from the F1
generation to self-pollinate
• After the seeds formed, he planted them
and counted more than 1000 plants in the
second generation or F2
• Mendel found that ¾ of the plants were tall
and ¼ of the offspring were short
Rule of Unit Factors
• Mendel concluded that each organism has
two factors that control each of its traits
• We now know that these factors are genes
and that they are located on chromosomes
• Genes exist in alternative forms, we call
these different gene forms alleles
• One allele is inherited from the mother and
one is from the father
Rule of Dominance
• In Mendel’s crosses, the observed trait
was referred to as dominant (T)
• The trait that seemed to disappear was
called recessive (t)
• Dominant alleles are capitalized and
recessive alleles are lower-case
Law of Segregation
• The law of segregation states that every
individual has two alleles of each gene
and when gametes are produced, each
gamete receives one of these alleles
• During fertilization, these gametes
randomly pair to produce four combos of
• The way an organism looks and behaves
is called its phenotypes
• Therefore, two organisms can look alike
but have different underlying allele
combos, Tt or TT, either one will display a
dominant phenotype
• The allele combination an organism
contains is known as its genotype
• The genotype of a tall plant that has two
alleles for tallness is TT
• The genotype of a tall plant that has one
allele for tallness and one for shortness is
• An organism’s genotype can not always
be seen
Homozygous or Heterozygous
• An organism is homozygous for a trait if its
two alleles for the trait are the same
-homozygous dominant (TT)- both code for
-homozygous recessive (tt)- both code for
• An organism is heterozygous for a trait if
its two alleles for the trait differ from each
other (Tt)
Mendel’s Dihybrid Crosses
• Mendel preformed another set of crosses
in which he used peas that differed from
each other in two traits rather than one
• A cross involving two different traits is
called a dihybrid cross
Parental Generation
• Mendel took true-breeding pea plants that
had round, yellow seeds (RRYY) and
crossed them with true-breeding pea
plants that had wrinkled, green seeds
First Generation
• He already knew that the F1 generation
would have all plants displaying the
dominant phenotypes; round, yellow seeds
Second Generation
• Mendel then let the F1 plants self-pollinate
• As expected the recessive reappeared
• When Mendel sorted and counted the
plants of the F2 generation, he found they
appeared in a definite ratio of phenotypes
-9 round yellow: 3 round green: 3 wrinkled
yellow: 1 wrinkled green
Law of Independent Assortment
• Mendel’s second law states that genes for
different traits- for example, seed shape
and seed color- are inherited
independently of each other
Punnett Squares
• In 1905, Reginald Punnett, an English
biologist, devised a shorthand way of
finding the expected proportions of
possible genotypes in the offspring of a
• This method is called a Punnett Square
• Punnett squares are good for showing all
the possible combos of gametes and the
likelihood that each will occur
• In reality, however, you don’t get the exact
ratio of results shown in the square
• Genetics is more like flipping a coin-it
follows the rules of chance