All humans and most life on earth are created from a genetic code in the form of deoxyribonucleic acid, more commonly known as DNA. In eukaryotes, DNA is located in the nucleus of a cell and the mitochondria.
Adenine, guanine, cytosine and thymine are the four chemical bases that form the foundation of all DNA. The thread-like structures that hold the DNA are called chromosomes.
Offspring Meaning in Biology
In biology, an offspring is the child of two organisms. Offspring contain characteristics of both parental organisms.
Plants, animals, fungi and bacteria reproduce in different ways to produce multiple offspring.
Human Heredity Basics
Humans reproduce sexually, meaning that each child is the combination both the mother and father's DNA. Humans have 23 pairs of chromosomes. For example, there are two forms of the sex chromosome in humans, X and Y. Males have an X and a Y chromosome while females have XX.
The father stores a single set of his chromosomes in each sperm, some will have the X chromosome and some will have the Y. The mother has a single set of her chromosomes in each of her eggs and as women have two X chromosomes, all her eggs will have the X chromosome. Since the sperm and egg sex cells only contain one set of chromosomes, they are called haploid sex cells.
When two haploid cells combine, they form diploid cells. The combination of two haploid chromosomes creates a unique genetic code, which is the code to grow the offspring through the replication of somatic cells. Somatic cells are the non-sex cells that make up our body such as fat, skin, muscle and blood cells.
Meiosis and Mitosis
Meiosis and mitosis are both forms of cell division. Mitosis is when a diploid cell creates a duplication of itself to form two new diploid cells. Meiosis is when diploid cells divide into haploid cells to produce sex cells for reproduction.
It is called genetic recombination when two haploid cells combine to create new diploid cells.
A phenotype is an observable physical and behavioral characteristic of an organism based on their genes. Each chromosome contains many different alleles which make up the code for different genes. Different allele combinations create different phenotypes.
Recombinant offspring are children that have a different allele combination to their parents.
For example, say a mother has a haploid cell with the alleles AB and the father has a haploid cell with the alleles ab. These combine to make a diploid cell with the sequence Aa+Bb.
Meiosis then produces four more haploid cells. The AB and ab haploid cells are the same as the parental type, while the Ab and aB are the recombinants due to the fact that they differ from the parental types.
Formation of Recombinant Offspring
Recombination can happen in two different ways; independent assortment and crossing over. Independent assortment is when the maternal and parental DNA are mixed during meiosis, creating a new gene sequence.
Crossing over happens during the first stage of meiosis when the two homologous chromosomes are paired and a portion breaks off on the same loci then reconnects to a different end. Crossing over can only happen when there isn't a physical linkage of the parental alleles.
Finding Recombinant Offspring
Recombination occurs when the number of switches between two loci is uneven. When looking for offspring with recombinant phenotypes, it is essential to remember that it is a comparison of the input from the parents with the output after meiosis. It is more straightforward to identify recombinants in haploid cells than diploid cells.
A testcross is required to analyze whether or not recombinant offspring are produced. When looking at a testcross, if the recombinant percentage is 50 percent, then independent assortment has occurred. When the recombinant rate is much less than 50 percent, this indicates a linkage and crossing over has occurred.
Example of Finding Recombinant Offspring
For example, say we have a mother plant with long pink flowers (AB) and a father plant of the same species with little white (ab) flowers.
In the example, the plants produce 100 offspring, 10 with long white (Ab) flowers, 8 with little pink (aB) flowers, 42 with long pink (AB) flowers and 40 with little white (ab) flowers. Of the offspring, 18 (or 18 percent) have a different phenotype from their parents as 18 divided by 100 is 0.18.
Since this number is much lower than 50 percent, it can be assumed that these offspring were likely created from crossover recombination.
- NIH: What is DNA?
- An Introduction to Genetic Analysis. 7th edition: Recombination
- Merriam Webster: Offspring
- NIH: What is a Chromosome
- Reference: What Is the Difference Between Diploid and Haploid Cells?
- ThoughtCo: 7 Differences Between Mitosis and Meiosis
- Reference: What is an Allele?
- NIH: How Many Chromosomes?
- Science Daily: Chromosomal Crossover
- Nature: Principle of Independent Assortment
- University of Washington: Genetic Linkage, and the Joint Inheritance of Traits
- Reference: What is the Difference Between Genotype and Phenotype
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