Observable Patterns of Inheritance

OBSERVABLE PATTERNS OF INHERITANCE

Cruciferous vegetables, like those pictured, can have a bitter taste. PTC is a chemical that tastes bitter, and is similar to the chemicals in cruciferous vegetables. Most people can taste the chemical PTC, but there are people who can’t taste it at all. If your parents can’t taste PTC, you are less likely to be able to taste PTC than someone whose parents can taste it.

 

1. Explain how your parents’ ability to taste PTC is a factor in your ability to taste PTC.
2. What are three questions we could investigate to better understand why a parent’s ability to taste PTC affects their children’s ability to taste it.
3. What other phenomena might be explained using the same science concepts as those you used to explain why only some people can taste PTC? 

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Bio.3.2 Trait Inheritance
Use computational thinking and patterns to make predictions about the expression of specific traits that are passed in genes on chromosomes from parents
to offspring. Emphasize that various inheritance patterns can be predicted by observing the way genes are expressed. Examples of tools to make predictions
could include Punnett squares, pedigrees, or karyotypes. Examples of allele crosses could include dominant/recessive, incomplete dominant, codominant, or
sex-linked alleles.

Genes and Proteins
In the previous section, you learned that DNA contains instructions to build the proteins that your cells need to function. An individual’s DNA is arranged in
chromosomes. Each chromosome includes a section of DNA wrapped around proteins, which keeps the DNA organized.

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We use patterns to make predictions about what traits offspring could inherit. As you read this chapter, look for ways that patterns can be used to predict which traits are passed on and expressed. 

Introns are areas of the DNA that are NOT used to make proteins.  Exons  ARE USED to make proteins.  mRNA transcribes both Introns and Exons, but enzymes snip out the Introns later.  Some Introns may be evoluntionary junk, past mutations that led nowhere.  Other introns  can be snipped apart and spliced back together in different ways.  This allows a cell  to produce different versions of a protein from the same gene. 

A section of DNA that codes for a protein is called a gene. Most genes contain the instructions for a single protein. There may be hundreds or even thousands of genes
on a single chromosome. Humans have about 22,000 genes. Everyone has the same genes, but due to small differences in the sequence of DNA bases, the proteins
produced from each gene may not be the same. The proteins that are made affect the traits that an individual has.

We can use observable patterns and our understanding of how DNA is passed from parent to offspring to make predictions about the likelihood of a trait being passed to
and expressed in an offspring.

The picture is called a Karyotype.  It is a picture of a persons chromosomes.  It numbers our chromosomes pairs 1-23, and shows the homologues ( Mothers and Fathers chromosome) next to each other.  Cells are stained and put under a microscope.  A picture of the chromosomes is taken with a microscope. The picture is cut up and the chromosomes arranged by matching size. They are arranged largest to smallest.  This one is a computer generated diagram, but most often you will see a Karyotype as a photo.

This diagram represents the 23 pairs of chromosomes in a human. Pairs 1-22 are labeled. These chromosomes are called autosomes. The 23rd pair could
include an X chromosome and a Y chromosome (for males), or two X chromosomes (for females). The 23rd pair of chromosomes determines the sex of the individual, and is called the sex chromosomes. One chromosome from each pair was inherited from the individual’s mother and the other
chromosome was inherited from the individual’s father. Except for the Y chromosome in males, the genes on each pair of chromosomes are the same. This means that an individual inherits two copies of each gene; one copy from their mother and one copy from their father. 

 A karyotype is useful in helping diagnose, learn about, and explain many genetic diseases. Sometimes during meiosis the chromosomes do not divide evenly or they are broken, and the offspring can inherit an abnormal number of chromosomes. Karyotypes allow us to see these abnormalities, and can be used to identify extra, defective, or missing chromosomes. Compare the two karyotypes below to determine which has the normal number of chromosomes and which has an abnormal number of chromosomes. 

Genes and Alleles
The chromosomes that make a pair are called homologous, because they contain the same genes. Even though the chromosomes contain the same genes, they are not identical . The majority of human genes have two or more possible alleles- which are alternative forms of a gene. For example, there are different versions, or alleles, of the gene that codes for the protein hemoglobin. Although each gene will produce the hemoglobin, small differences in the DNA can cause the hemoglobin protein to have a different shape. One allele for hemoglobin has one different base, and the changes to the protein result in sickle cell anemia. 

People with sickle cell anemia can experience a range of symptoms, including reduced blood flow to tissues, pain, and the death of cells. The differently-shaped
hemoglobin protein causes red blood cells to change to a sickle shape. The images show normal and sickle shaped cells, and how the sickle shaped cells can interrupt the flow of blood. People with two copies of the sickle cell allele will have sickle cell
anemia, but people who inherit one or two copies of the normal allele will not have
sickle cell anemia.

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Most human genetic variation is the result of differences in individual DNA bases within alleles. The combination of alleles that an individual inherits from their parents determines what traits they have. Some alleles are always expressed, meaning that the trait associated with the allele will be present. These alleles are sometimes called “dominant” alleles, and they are represented using a capital letter. Other alleles may not be expressed. These alleles are sometimes called “recessive” alleles, and they are represented using a lower case letter. Even though an individual has a recessive allele, they will not have the trait that the allele codes for, but they could pass the allele to their offspring, and their offspring could have the trait.

The terms “dominant” and “recessive” can be misleading. Dominant alleles are not necessarily more common than recessive alleles, and they are not more likely to be passed on than recessive alleles. Recessive alleles are not always hidden. There are cases when alleles do not follow a dominant-recessive pattern. 

PATTERNS OF INHERITANCE

We can use patterns in the expression of traits to learn if an allele is dominant, recessive, or neither, and to make predictions about the likelihood of a particular trait being expressed in the offspring. Before using patterns to make predictions, it is important to understand the relationship between the alleles an individual has and the traits they express. The combination of alleles an individual has is their genotype, and the expression of traits
is their phenotype. As an example, assume that the allele for brown fur (A) is dominant to the allele for gray fur (a). The table below shows the possible combinations of alleles that an individual could have, and the traits associated with each allele combination.

PHENOTYPE- the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.

GENOTYPE:  the genetic arrangement that makes up the traits that an organism inherited from its parents.

Bb= Brown eyes          

Phenotype:  

Genotype	Phenotype
bb	Blue Eyes
Bb, bB	Brown Eyes

Here lower case "b" represents the recessive trait of blue eyes.  The upper case "B" represents the dominant trait of brown eyes.

Lets say we are looking at rabbits.  What is the possibility of two brown individuals having a gray offspring? Or of brown and gray parents having brown offspring? We can use observable patterns to make predictions about the possible genotype and phenotype of the offspring. This is important when it comes to breeding animals and plants.

Gregor Mendel is known as the Father of Genetics, because his work established a foundation for understanding genetics. Gregor Mendel was a Czech and a Monk, and Abbot of St. Thomas Abby in Brunn.  He went to college and studied meteorology, mathematics and biology.  One of his greatest influences, though, was growing up a farm boy.  Farmers had studied crossing various plants and animals to get certain traits for over 1000 years.  Most of rest of the population had the idea that  the traits of two parents would mix together.....in other words a black horse and a white horse mated, would have offspring that was grey.  This is not true.  We would all look alike within just a few generations if that were the case. 

Mendel studied inheritance patterns in pea plants from 1856 to1863. Mendelian inheritance refers to the inheritance of traits controlled by a single gene with two alleles, one of which may be dominant to the other. Mendel discovered that genes come in pairs, and that these gene pairs are separated and only one gene from each pair is passed to an offspring.  He studied seven traits of pea plants; Plant height, pod color and shape, pea color and shape, and flower position and color. He did not know about genes, but said that the parent plants had "factors" they passed along to offspring. He published his work in 1866, and it was basically lost and forgotten, until rediscovered after his death in 1900.  He lectured a few times about it, but other scientists of the time failed to appreciate its significance. It is lucky he studied these traits in pea plants, single gene traits with fully dominant and recessive alleles.  Had he studied some other trait, that is controlled by more than one gene, or not fully dominant or recessive, or sex linked, he may never have made his discoveries.  Mendel created the Punnet Square, a method of figuring out the likelihood of a trait being expressed.

A Punnett square is a chart that allows you to easily determine the expected percentage of different genotypes in the offspring of two parents. An example of a Punnett square for pea plants is shown. In this example, both parents have the genotype Bb for flower color. This means that the homologous chromosomes that include the gene for color have different alleles. These chromosomes will be separated during meiosis, and the parent will only pass one chromosome along to each offspring. The gametes produced by the male parent are at the top of the chart, and the gametes produced by the female parent are along the side. The different possible combinations of alleles in their offspring are determined by filling in the spaces of the Punnett square with the correct alleles. 

 

Predicting Offspring Genotypes

Understand these terms:

Heterozygous- having two different alleles- such as Bb 

Homozygous- having two identical alleles- such as BB or bb

Monohybrid cross- a cross focusing on one trait

In the cross shown in the figure, you can see that one out of four offspring (25 percent) has the genotype BB, one out of four (25 percent) has the genotype bb, and two out of four (50 percent) have the genotype Bb. These percentages of genotypes are what you would expect in any cross between two heterozygous parents. If these parents only had four offspring, they won’t necessarily have one offspring with genotype BB, two with Bb, and one with bb. Each offspring has the same chance of being BB (or Bb or bb). However, if you considered hundreds of such crosses and thousands of offspring, you would get very close to the expected results, just like tossing a coin. 

You can predict the percentages of phenotypes in the offspring of this cross from their genotypes. B is dominant to b, so offspring with either the BB or Bb genotype will have the purple-flower phenotype. Only offspring with the bb genotype will have the white-flower phenotype. Therefore, in this cross, you would expect three out of four (75 percent) of the offspring to have purple flowers and one out of four (25 percent) to have white flowers. These are the same percentages that Mendel got when experimenting with pea plants. 

This video will help you understand Punnet Squares:

https://www.youtube.com/watch?v=i-0rSv6oxSY&list=PLwL0Myd7Dk1FVxYPO_bVbk8oOD5EZ2o5W&index=7

Determining Missing Genotypes
A Punnett square can also be used to determine a missing genotype based on the other genotypes involved in a cross. Suppose you have a parent plant with purple flowers and a parent plant with white flowers. Because the b allele is recessive, you know that the white-flowered parent must have the genotype bb. The purple-flowered parent, on the other hand, could have either the BB or the Bb genotype. The Punnett square below shows this cross. The question marks (?) in the chart could be either B or b alleles.

Can you tell what the genotype of the purple-flowered parent is from the information in the Punnett square? No; you also need to know the genotypes of the offspring in row 2. What if you found out that two of the four offspring have white flowers? Now you know that the offspring in the second row must have the bb genotype. One of their b alleles obviously comes from the white-flowered (bb) parent, because that’s the only allele this parent has. The other b allele must come from the purple-flowered parent. Therefore, the parent with purple flowers must have the genotype Bb. Not many human traits are controlled by a single gene with two alleles, but this type of inheritance is a good starting point for understanding human heredity. How Mendelian traits are inherited depends on whether the traits are controlled by genes on autosomes or the sex chromosomes.

Autosomal Traits

Autosome- any chromosome that IS NOT a sex chromosome

Autosomal traits are controlled by genes on one of the 22 human autosomes. Consider earlobe attachment. A single autosomal gene with two alleles determines
whether you have attached earlobes or free-hanging earlobes. The allele for free-hanging earlobes (F) is dominant to the allele for attached earlobes (f). Other
single-gene autosomal traits include widow’s peak and hitchhiker’s thumb. The dominant and recessive forms of these traits are shown in the image. Which form of these traits do you have? What are your possible genotypes for the traits?