Inheritance and Variation of Traits

In this lesson, students focus on the science phenomenon of reproduction, using Punnett squares and other models to figure out the different advantages between asexual and sexual reproduction. They use their investigative data to support an explanation about the evolutionary benefits of each kind of reproduction. 

• Reproduction is an important part of heredity because it is how parents pass traits onto offspring. As long as organisms successfully reproduce, they will not become extinct. 
• Scientists can use models called Punnett squares to visually represent the likelihood of offspring inheriting particular traits in sexual reproduction.
• Offspring of sexually reproducing organisms inherit 50 percent of their genes from their mother and 50 percent of their genes from their father. Each parent contributes one allele for each trait. This means that each offspring receives a different combination of genes from their parents. Alleles can be dominant or recessive.
• DNA is organized into genes, which hold the information necessary for building all of the proteins needed to keep an organism functioning. The DNA is held in chromosomes, which are passed down from parents to offspring when parents reproduce.
• Asexual reproduction is more efficient, so there has to be some evolutionary benefit that organisms get from sexual reproduction because it is so common among species.
• Scientists think that the genetic variation of sexual reproduction makes species more able to adapt to changes in their environment. This would make them more likely to survive as a species.
  
  

• How is reproduction related to DNA?
• What is the biggest difference between asexual and sexual reproduction?
• How do chromosomes get passed on in asexual compared to sexual reproduction?
• Why are the offspring of sexually reproducing organisms different from their parents?
• How similar are the offspring of asexually reproducing organisms to their parents?
• How are dominant alleles different from recessive alleles?
• How are the genotypes of the parents displayed in the Punnett square?
• How can we predict the probability of an offspring plant of these two parents being tall or short?
• Why is asexual reproduction more efficient than sexual reproduction?
• Why do scientists think that sexual reproduction is beneficial to organisms?
  
  

Misconception: In natural selection, organisms “try” to adapt so they can get what they need.

Fact: The genetic variations that lead to natural selection occur randomly. Natural selection also depends on the environment because the organisms with genes best suited to their environment will survive and reproduce.

Allele : a form of the same gene that has small differences in the sequence of DNA bases

Asexual Reproduction :  reproduction that requires only one parent (e.g., binary fission, budding, and fragmentation)

Chromosome : a threadlike structure of DNA and protein; found in the nucleus of eukaryotic cells; a discrete package of genetic material

Reproduction :  the ability of a mature organism to have offspring

Sexual Reproduction : the creation of a new individual from combined genetic information of two parents of different sexes

The Birth of a Panda

In August 2015, a giant panda named Mei Xiang gave birth. She had two babies at the Smithsonian National Zoo. This was an exciting event at the zoo. Almost 900,000 people watched the zoo’s panda cam over the weekend of the birth

There has been a lot of effort aimed at getting pandas to give birth and raise healthy cubs in captivity. Breeding giant pandas in captivity isn’t easy, but people around the world are working on this.

The reason so many people are trying to breed giant pandas is that they are an endangered species. Endangered species are seriously at risk of becoming extinct. Remember that extinction happens when there are no longer any living members of a species.

The Importance of Reproduction

As long as organisms successfully reproduce, they will not become extinct. Reproduction is the ability of a mature organism to have offspring. Reproduction is an important part of heredity. Heredity is the passing on of traits from parents to offspring.

For giant pandas to reproduce, they need both a male and a female panda. This is because giant pandas reproduce sexually. Sexual reproduction creates offspring with combined genetic information of a male and a female. The combination of genes ensures that each offspring has a different set of traits.

Sexual Reproduction

To understand how sexual reproduction happens between two organisms, we need to return to DNA. Remember that DNA is organized into genes. Genes hold the information necessary for building all of the proteins needed to keep an organism functioning. DNA floats in the cytoplasm of prokaryotes. It is held in the nucleus of eukaryotes.

In the cell, DNA condenses into threadlike structures called chromosomes. Each chromosome is made up of a single DNA molecule. It holds hundreds or thousands of genes on it. The genes are located on the chromosomes in a very specific way. Because of this, if scientists know where one gene is located, they can find it on anyone’s chromosomes. Each chromosome is made up of a single DNA molecule, and it holds hundreds or thousands of genes on it.

Fertilization

In humans, there are 23 pairs of chromosomes. Chromosomes are in pairs because one chromosome in each pair is inherited from each parent. In humans and many other kinds of multi- celled organisms, each parent produces cells that have half of the chromosomes as the parent. In males, these cells are called sperm cells. In females, they are called eggs. A sperm and an egg join through a process called fertilization. When they join, they form a cell that has a complete set of chromosomes, one half from each parent.

Both chromosomes in a chromosome pair contain genes that code for the same proteins. These are called alleles. An allele is a form of the same gene that has small differences in the sequence of DNA bases. For example, one allele might have the instructions for proteins that would result in blue eyes. Another allele might have the instructions for proteins that would result in brown eyes.

In sexually reproducing organisms, each parent contributes one allele for each trait. This means that the offspring have two sources of genetic information. One source comes from each parent. When baby pandas are born, they have the combined genetic information of both parents. This means they share some traits with each of their parents. However, they aren’t genetically identical to either one.

Dominant and Recessive Genes

The genetic diversity caused by sexual reproduction explains why siblings look similar to each other but also have some differences. Part of this has to do with how the two alleles of a gene interact.

An allele can be dominant or recessive. We can use eye color in sexually reproducing organisms to explore this idea more. A dominant allele shows its effect even if the organism just has one copy of it. For example, the allele for brown eyes is dominant. A person needs to inherit just one allele for brown eyes from one of their parents to have brown eyes.

In contrast, recessive alleles only show their effect if the organism has two copies of it. You would need to inherit recessive alleles from both parents to have gray, green, hazel, or blue eyes.

In this lesson, students figure out the advantages and disadvantages of the phenomena of sexual and asexual reproduction by experimenting with the traits of fish as they pass on their genes to offspring in a hostile environment. Through their investigations, students use what they know about heredity and reproduction to see how sexual reproduction happens slowly and introduces genetic variation into a population, while asexual reproduction happens quickly and produces genetically identical offspring.