Michael G. answered • 05/18/20
Experienced Genetics & Molecular Biology Tutor
This is a really thorough question, that I personally found really cool to see some data on it.
So first look on the second column of the left side of the table, which tells you the traits. It appears that there are a number of genes that correspond with very short, short, average height, etc.
Now look to the right of this column, & you see the percentage of offspring produced. The trait with the greatest percentage of offspring produced is the Average height trait, & the percentage of offspring born with the specified trait tapers off as you move towards the extreme. So what I mean by that is that as you move to very tall & very short, the percentage of offspring who fall into the category drop off. If you graphed this data you would get a graph that looks like it has a big hill in the middle, & we call these types of graphs Normal curves.
Before continuing my explanation I just want to cover some terminology to help understand what these things mean. An allele is a variation of a gene. For example, if a gene for eye color is being used, a person could have a green allele, a blue allele, or a brown allele. Polygenic traits are traits that are influenced by multiple genes. These would be complex traits, such as height, or intelligence, because it's not as simple as a person being tall or short, they could be somewhere in the middle & as we see with this table height is encoded by many genes (probably more than what are listed).
These genes are most likely spread out throughout the genome, meaning some are on chromosome 1, some are on chromosome 14, & many others. When two parents mate, & produce many offspring you can still see variations in height that were passed down. This is all because of genetic diversity, which is essential for evolution. There are many ways to diversify genes that are passed down, & they occur during meiosis, which is the process that produces gametes (Sperm cells & egg cells).
In prophase 1 of meiosis, we have a process called crossing over. This process describes the shuffling of homologous genes. Each person has two copies of each chromosome, & each copy is capable of swapping alleles for each gene. This makes it so that the chromosome that is inherited is a combination of the grandparent's genes (This part is confusing so look at some images if that confused you). As you continue through meiosis you will run into another process that gives genetic diversity called independent assortment. During metaphase 1 & 2, the chromosomes line up in the middle of the cell. The chromosomes are about to be pulled apart into different cells, so the way they line up dictates which chromosomes are being put into daughter cell 1 or cell 2. Genes on different chromosomes sort independently, meaning the chances that a tall allele going into cell 1 is not changed by where the allele for eye color goes, as long as they are on different chromosomes (That last part is also confusing, so another image may help understanding).
So relating back to your question, these meiotic processes for genetic diversity greatly vary the genes that are passed on, so the children will inherit different alleles. For polygenic traits, such as height, it makes it less likely that the exact same combinations of genes are shared in siblings. The only time two siblings would get the same traits are in identical twins, or in a 1/42 chance.
Wow I just went through a lot there. I hope this helps you. If you liked my explanation you should sign up for tutoring with me & I can walk you through many similar processes to increase your understanding.
Kindly,
Michael Greenberg
