Heredity is the genetic makeup that's passed on from parents to offspring. It influences our physical characteristics, but can also affect behavior.
So we're going to look at how our parents pass on their genetic information to us, and how genes influence what we look like and how we act. Genes may influence one to behave in certain ways, such as cheerfully, aggressively, or irrationally.
Other behavioral traits that are influenced by genes include right- and left-handedness and stuttering. There's no single gene responsible for one's behavior and temperament. Rather, it's an interaction of one's genes and one's environment that influence how one thinks, acts, and feels. So if you look at this picture of a child, he may be genetically more predisposed to temper tantrums, but his environment triggers him to behave that way.
So let's take a look at our genetic makeup and the role of chromosomes, genes, and DNA. Chromosomes are passed from parents to children. They're thread-like strands of DNA molecules that contain genetic information.
Humans have 46, half from each parent, and they're arranged in 23 pairs. One pair is made up of the sex chromosomes. So chromosomes are like a genetic blueprint. They influence how you look and act. All living things are made up of deoxyribonucleic acid, or DNA. It's a complex molecule that contains the genetic information that makes up chromosomes.
It's made up of four chemical bases, adenine, cytosine, guanine, and thymine. About 99.9% of our DNA is shared. So we're close to identical twins with everyone else on earth. But that 0.1% and our environments make us vastly different people. The chemical bases arrange themselves into pairs.
There are thousands of different possible combinations in each gene, and arrangement of the bases is called the genetic code. Everyone's genetic code is unique, aside from identical twins. Each chromosome has thousands of DNA segments, called genes. That's a segment of DNA coded for specific hereditary traits.
It's located inside a chromosome and serves as a biochemical unit of heredity. They operate in pairs, like chromosomes, so a pair of genes can affect a particular process or a personality characteristic. Look at the image of DNA that's formed from several books. Genes, like each book that's stacked together, make up DNA.
A majority of human traits are polygenic. That's when a trait is influenced by a combination of multiple genes. So a combination of genes can affect our height, eye color, or skin tone. When we're talking about height, we inherit the potential to be a certain height, but environmental factors influence our actual height that we attain.
Environmental factors can affect how a trait is expressed.
Genetic factors increase the likelihood that certain behavior, abilities, or personality traits emerge, but whether or not they do depends on environment. Scientists may use a diagram called a Punnett Square. It's used to determine the probability and possible outcomes of a genetic cross, and allows one to see what traits may be inherited.
So in this example, we can see what will happen if a yellow pea plant is crossed with a green pea plant. The Punnett Square lets you see all the possible outcomes of this cross.
Alleles are the different forms of a gene that are responsible for hereditary variation. If it's dominant, the gene is expressed each time it is present. Recessive means that the gene is expressed only when paired with another recessive gene.
In this example, the dominant allele is the yellow capital letter Y. The recessive allele is the green lowercase letter Y. Whatever trait the dominant allele represents is going to be expressed. Here are some examples of dominant and recessive genes.
Dominant genes can result in a cleft chin, widow's peak, free earlobes, or dimples. Recessive can result in no cleft chin, no widow's peak, attached earlobes, and no dimples. Just because a gene is dominant doesn't mean that it's common, however. The gene for six fingers is dominant and the gene for five fingers is recessive.
Genotype is our genetic makeup. That's all possible combinations of alleles. Phenotype is the physical expression of the trait.
So here's an example of a Punnett Square talking about sex characteristics. If we have the sex chromosomes, the female egg cells would be two X chromosomes, and the male sperm cell would be an X and a Y chromosome.
The sex cells unite to determine the gender, which is controlled by the type of sperm.
So if you have two X, it's going to result in a female. If you have an X and a Y, it's going to result in a male. So the genotype is either XX or XY, and the phenotype is either girl or boy.
Let's take a look at another example. In this case, we're going to discuss the dominant trait of a widow's peak, which is when your hair comes to a point on your forehead. If there is not a dominant trait, then your hairline is going to be straight across.
In this case, we've use the capital H to represent the dominant trait, and the lowercase H to represent the recessive trait.
In this example, the male and the female each had a dominant and recessive trait. So let's look at all the possible outcomes for this cross.
You can have two capital Hs, which are dominant, and get a widow's peak. A capital H and a lowercase H will give you a widow's peak. And two lowercase Hs, or two recessive traits, will give you a hairline that's straight across.
So the genotypes of either two capital Hs, a capital H and a lowercase H, or two lowercase Hs, will give you the phenotype of either a widow's peak or no widow's peak.
Studies are used by researchers to determine the impact of hereditary and genes. Some of those studies include family studies. They study the resemblances among blood relatives, and can indicate whether a trait runs in the family.
Twin studies study identical and fraternal twins and compare the similarities. And adoption studies study resemblances among adopted children and their biological and adoptive parents.