Inheritance Chapter Notes | Science for Grade 6 PDF Download

Introduction

The chapter "Inheritance" helps us understand why some children look like their parents while others do not. It explains how traits, like eye color or height, are passed from parents to their kids through something called heredity. This chapter also talks about the work of Gregor Mendel, a scientist who studied how traits are inherited by experimenting with pea plants. You will learn about genes, chromosomes, and how tools like Punnett squares and pedigrees help us predict and understand traits in families. By the end, you’ll know how traits are passed down and why some traits show up while others stay hidden.

What are traits?

• Traits are special features that make each person unique, like hair color or height.
• The way a trait looks is called its phenotype.
• Some traits, like eye color, have many types, while others, like earlobe shape, have only two types.
• Traits are passed from parents to their children through heredity.
• Heredity is the process of passing traits from parents to their offspring.
• Sometimes, a child can have a trait different from their parents, like straight thumbs when both parents have curved thumbs.

Why do offspring look like their parents?

Offspring often look like their parents because they inherit traits from them.

First-Generation Crosses

• When Mendel crossed two true-breeding plants with purple flowers, all offspring had purple flowers.
• When he crossed two true-breeding plants with white flowers, all offspring had white flowers.
• When he crossed a true-breeding purple-flowered plant with a true-breeding white-flowered plant, all offspring had purple flowers.
• This was surprising because Mendel expected a mix of purple and white, like light purple, but only purple flowers appeared.
• The offspring from this cross are called hybrid plants because they come from parents with different forms of the same trait.

Second-Generation (Hybrid) Crosses

• Mendel then crossed two purple-flowering hybrid plants to see what would happen.
• The offspring had both purple and white flowers, even though both parents had purple flowers.
• This showed that the white flower trait, which disappeared in the first generation, could reappear in the second generation.

Reappearing Traits

• Mendel noticed that when he crossed hybrid plants, some traits that disappeared in the first generation came back in the second generation.
• For example, white flowers reappeared in the offspring of two purple-flowering hybrid plants.
• This pattern was the same for other traits Mendel studied in pea plants.

A Similar Relationship

• Mendel found that when he crossed hybrid plants with purple flowers, about three times as many offspring had purple flowers as had white flowers.
• This 3 : 1 ratio was the same for all traits he tested in pea plants.
• Mendel suggested that hybrid plants have one genetic factor for purple flowers and one for white flowers.
• He thought the purple flower factor was stronger and blocked the white flower factor.

Dominant and Recessive Traits

• A dominant trait is a genetic factor that shows up in offspring even if only one copy is present.
  • For example, purple flowers are dominant in pea plants.
• A recessive trait only shows up if the offspring has two copies of that trait.
  • For example, white flowers are recessive and only appear if the plant has two recessive factors.
• A dominant trait, like purple flowers, appears if the plant has one or two dominant factors.
• A recessive trait, like white flowers, only appears if the plant has two recessive factors.

Did You Know?

Gregor Mendel

• Gregor Mendel, an Austrian monk, studied how traits are passed down over 150 years ago.
• Mendel is known as the father of genetics, which is the study of how traits are inherited.
• Mendel used pea plants for his experiments because they were easy to study.
• He worked with true-breeding plants, which always produce offspring with the same traits as the parent when self-pollinated.
• For example, a true-breeding pea plant with wrinkled seeds only produces plants with wrinkled seeds when it self-pollinates.
• Mendel cross-pollinated true-breeding plants with different traits, like purple flowers and white flowers, to see what happened.

What controls traits?

• Scientists found that traits are controlled by genetic information inside the nucleus of cells.
• The nucleus contains threadlike structures called chromosomes.
• Chromosomes carry genetic information that controls traits.
• Each cell in offspring has chromosomes from both parents, forming pairs.
  • A gene is a part of a chromosome that controls one specific trait, like flower color.
  • Genes can have different forms called alleles, such as purple or white for flower color.
  • The two alleles for a trait make up its genotype, which determines how the trait looks (phenotype).
• Scientists use letters to show alleles: uppercase (like R) for dominant alleles and lowercase (like r) for recessive alleles.
  • A homozygous genotype means the two alleles are the same, like RR or rr.
  • A heterozygous genotype means the two alleles are different, like Rr.
  • For example, round seeds in peas can have genotypes RR (homozygous dominant) or Rr (heterozygous), and both look round.
  • Wrinkled seeds only have the genotype rr (homozygous recessive).

How can you predict what offspring will look like?

You can predict traits in offspring by comparing ratios, like flipping a coin where heads and tails have a 1 : 1 ratio.

Punnett Squares

• A Punnett square is a tool used to predict the possible genotypes and phenotypes of offspring.
• It shows how alleles from parents combine to form the offspring’s genotype.
• For example, crossing two hybrid plants with yellow seeds (Yy) can produce offspring with genotypes YY, Yy, or yy.
• Geneticists use Punnett squares to understand how traits are passed from one generation to the next.

Did You Know?

• A ratio compares two numbers, like the number of yellow seeds to green seeds in Mendel’s experiments.
• For example, Mendel found 6,022 yellow seeds and 2,001 green seeds, which simplifies to a 3 : 1 ratio.
  • This means offspring from heterozygous parents have a 3 : 1 chance of having yellow seeds.
  • Each offspring’s traits are independent, like how one coin toss doesn’t affect the next.

How can you model a family’s phenotypes?

• A pedigree is a chart that shows the phenotypes of family members who are genetically related. It helps figure out the genotypes of family members.
• For example, if offspring have attached earlobes (recessive, uu) but their parents have unattached earlobes, the parents must carry the recessive allele. This means the parents’ genotype is likely heterozygous (Uu), as they don’t show the recessive trait but can pass it on.