unit 2 lesson 4 - heredity - epiphany catholic school...2015/05/07 · unit 2 lesson 4 heredity how...

Unit 2 Lesson 4 - Heredity

7th Grade Cells and Heredity (Mod

A) Unit 2 Lesson 4 - Heredity

Give Peas a Chance

Copyright © Houghton Mifflin Harcourt Publishing Company

What is heredity?

• Traits, such as hair color, result from the information stored in genetic material.

• Heredity is the passing of genetic material from parents to offspring.

Unit 2 Lesson 4 Heredity

What did Gregor Mendel discover about heredity?

• Gregor Mendel was an Austrian monk. In the 1800s, Mendel performed the first major experiments in heredity.

• Mendel studied seven characteristics of pea plants.

• A characteristic is a feature that has different forms in a population.




• The seven different characteristics Mendel studied were plant height, flower and pod position, seed shape, seed color, pod shape, pod color, and flower color.

• Each characteristic had two different forms. These different forms are called traits.




• Mendel studied each characteristic separately, always starting with plants that were true-breeding.

• True-breeding plants always produce offspring with the same trait if allowed to self-pollinate naturally.




• Mendel crossed plants that were true-breeding for producing yellow seed pods with plants that were true-breeding for green seed pods.

• All of the plants from the first generation produced green seed pods.

• Mendel called the green seed pod the dominanttrait, and the yellow seed pod the recessive trait.




• Next, Mendel let the first generation plants self-pollinate.

• Out of the generation that resulted, called the second generation, about three-fourths had green seed pods and one-fourth had yellow pods.




• The recessive trait had seemed to disappear in the first generation, but it reappeared in the second generation.




• Mendel hypothesized that each plant must have two heritable “factors” for each trait, one from each parent.

• Some traits, such as yellow color, could only be observed if a plant had two of the same factors.

• A plant with two different factors would show the dominant factor but be able to pass on both factors to its offspring.



Do p. 124 & 125 #5 &7

It’s in Your Genes!


How are traits inherited?

• Mendel’s ideas can be further explained by our modern understanding of DNA.

• What Mendel called “factors” are actually segments of DNA known as genes.



• Genes are segments of DNA. They give instructions for producing a certain characteristic.



• The offspring has two versions of the same gene for every characteristic—one from each parent.

• Different versions of a gene are known as alleles.

• Dominant alleles are shown with a capital letter, and recessive alleles are shown with a lowercase version of the same letter.




• An organism with one dominant and one recessive allele for a gene is heterozygous for that gene.

• An organism with two of the same alleles for a gene is homozygous for that gene.




• The combination of alleles that you inherited from your parents is your genotype.

• Your observable traits make up your phenotype.

• The phenotypes of some traits follow patterns similar to the ones Mendel discovered.




• The dominant allele contributes to the phenotype if one or two copies are present in the genotype.

• The recessive allele contributes to the phenotype only when two copies of it are present.




• If one chromosome in the pair contains a dominant allele and the other contains a recessive allele, the dominant allele determines the phenotype.

• This is called complete dominance.




• Some characteristics are a result of several genes acting together.

• Sometimes, one gene influences more than one trait.

• For example, many genetic disorders, such as sickle cell anemia, are linked to a single gene but affect many traits.




• Sometimes, the environment can influence an organism’s phenotype.

• Some traits are acquired only from one’s environment and are not inherited.

• For example, your ability to read and write is an acquired trait.



Bending the Rules


What are the exceptions to complete dominance?

• Some traits do not follow the pattern of complete dominance.

• For traits that show incomplete dominance and codominance, one trait is not completely dominant over another.



• In incomplete dominance, each allele in a heterozygous individual influences the phenotype.

• The result of incomplete dominance is a phenotype that is a blend of the phenotypes of the parents.

• An example of this in humans is hair. A person with one allele for straight hair and one allele for curly hair will have wavy hair.




• For a trait that shows codominance, both of the alleles in a heterozygous individual contribute to the phenotype.

• Heterozygous individuals have both of the traits associated with their two alleles.




• Human blood type is an example of codominance.

• Three alleles, called A, B, and O, play a role in determining blood type.

• A person with an A allele and a B allele has type AB blood.



unit 2 lesson 4 - heredity - epiphany catholic school...2015/05/07 · unit 2 lesson 4 heredity how...

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