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Heredity Mendel and His Peas. Chapter 5. Heredity: the passing of traits from parents to offspring. Example: you might have curly hair, while both of your parents have straight hair or you might have blue eyes even though both of your parents have brown eyes. HOW IS THIS? Gregor Mendel:

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Heredity: the passing of traits from parents to offspring.

    • Example: you might have curly hair, while both of your parents have straight hair or you might have blue eyes even though both of your parents have brown eyes. HOW IS THIS?
  • Gregor Mendel:
    • Born in 1822
    • Austria
    • Grew up on farm and learned all about flowers and fruit trees
mendel continued
Mendel continued….
  • Entered a monastery when he was 21, where monks taught science and performed experiments.
  • He did well in school but he did not pass his final exam; therefore, he went back to the monastery.
  • While at the monastery he discovered the principles of heredity.

Mendel studied plant inheritance and tried to understand it,

  • he realized that some traits would show up in the first generation and then not in the second generation.
  • When developing an experiment Mendel decided to study only one organism.
  • The organism Mendel used was the pea plant because he had studied it before.
pea plants
Pea Plants
  • Mendel used the pea plant for several reasons.
    • Grow quickly
    • Many different kinds available
    • Able to self-pollinate
    • Able to cross-pollinate

Mendel only studied one characteristic at a time. (characteristic: a feature that has different forms in a population).

  • The different forms are called traits
  • The characteristics that Mendel studied were
    • Seed shape
    • Plant height
    • Flower color
first experiment
First Experiment
  • Mendel crossed pea plants to study seven different characteristics.
  • He crossed plants that had purple flowers with plants that had white flowers (figure 5 p 117).
  • The offspring, first generation, had all purple flowers. (first generation- self-pollination)
  • Mendel wondered why all the flowers came out purple when he started out with a white flower and a purple flower. What happened to the traits of the white flower?

Mendel discovered that all the traits that were expressed (shown) were called dominant traits and the traits that did not show up were called recessive traits.

  • Recede means “to go away or back off”
  • Mendel theorized that only certain traits would be expressed when combined with other traits.
second experiment
Second Experiment
  • Mendel did the same experiment on each of the seven characteristics.
  • In second generation plants, some expressed the recessive traits.
  • Mendel decided to count the number of plants with each trait that turned up in the second generation. (Table 1 p 118)
  • The dominant trait showed up more than the recessive trait.
  • Mendel then figured out the ratios of dominant traits to recessive traits. (ratio-a relationship between 2 different numbers that is often expressed as a fraction).
  • Math practice: p 118
traits and inheritance
Traits and Inheritance
  • After Mendel’s experiments he concluded that there are two sets of instructions for each characteristic.
  • Scientists call these instructions for an inherited trait genes.
  • Each parent gives one set of genes to the offspring giving the offspring two forms of the same gene fro every characteristic.
  • The different forms (dominant and recessive) of gene are known as alleles.
  • Dominant-capital letter, Recessive-lowercase letter

An organism’s appearance is known as its phenotype, what it looks like.

    • For example: the pea plants could either have a purple flowers or white flowers that would be the phenotype.
  • Albinism prevents hair, skin, and eyes from having normal coloring; therefore affecting the appearance of a person.

The inherited alleles from each parent come together to form an organism’s genotype.

  • Remember that dominant traits are capital letters and recessive traits are lowercase letters. So in the pea plants the allele for purple flowers is (P)-dominant, and the allele for white flowers is (p)-recessive.
  • So if the pea plant has two dominant alleles then its genotype would be (PP), if it had two recessive alleles then its phenotype would be (pp).
homozygous vs heterozygous
Homozygous vs. Heterozygous
  • If an organism has the genotype (PP) or (pp) it is said to be homozygous because it has the same two alleles.
  • If an organism has the genotype (Pp) it is said to be heterozygous because it has two different alleles. When this happens the dominant trait is expressed and the recessive trait is masked by the dominant allele.
punnett square
Punnett Square
  • A punnett square is used to organize all possible combinations of offspring from particular parents.

p p



  • All offspring will be heterozygous dominant for purple

In Mendel’s experiments the first generation was self-pollinated making all offspring (Pp).

  • In the second generation he crossed a (Pp) and a (Pp)
  • Make the punnett square for the outcome of the offspring.
  • What would the ratio of purple flowers to white flowers be?
  • The mathematical chance that something will happen is known as probability.
  • It is usually written as either a fraction or percentage.
  • Example: when you toss a coin there is a 50 % chance that is will land on heads and a 50 % chance it will land on tails.
  • So when you cross a (Pp) and (Pp) the probability will be …___%_ purple and ___%_white
incomplete dominance
Incomplete Dominance
  • Scientists have discovered that some traits are not completely dominant over another trait. Therefore each trait influences that outcome of the offspring. This is called incomplete dominance.
  • When crossing snapdragons( figure 5 p 124) one being white and one being red the offspring outcome is 4 pink snapdragons instead of 3 red and 1 white.
genes and traits
Genes and Traits
  • Sometimes one gene can code for many traits. For instance the tiger in figure 6 appears to be white, but it also has blue eyes. One gene controlled those two traits.
  • Other times many genes can code for the same trait. At least two genes determine human eye color. That’s why many shades of a single color are possible.
  • The environment can also effect the outcomes. A guinea pig could have genes for long hair but its fur could be cut. Also height can be controlled by a healthy diet.
me iosis
  • Sexual reproduction- two parents
  • Homologous chromosomes- same genes and same structure.
  • Meiosis- forming of sex cells, producing cells with half the number of chromosomes. (23-mom, 23-dad = 46)
  • Male-sperm, Female-egg
  • Genes are located on chromosomes
sex chromosome
Sex Chromosome
  • Female-XX X Y
  • Male-XY X


  • So what is the probability of the offspring being a girl? Boy?
  • What is the genotype for a boy? girl?
  • Read Sex-Linked Disorders p 131
sex linked disorders
Sex-Linked Disorders
  • Colorblindness -males
  • Hemophilia- prevents blood from clotting.
  • Genetic disorders can be traced through a family tree. Counselors use a diagram called a pedigree. This helps to see the outcomes of future generations.
  • Some diseases that are traced:
    • Cystic fibrosis-lung problems (recessive trait)