Pedigree and Genetic Disorder Study Guide

Pedigrees and Genetic Disorders Study Guide
When you are talking about a pedigree dog, it means the dog is purebred. Through selective breeding, the dog has all the traits of that particular breed. 

We

use a very similar chart  to record and research our own genealogy. Here is the genealogy chart of President Theodore Roosevelt and President Franklin Delano Roosevelt, 26th and 32nd Presidents of the united states. 

When talking about genetics, however, a pedigree is a chart that helps show family relationships. Scientists use standardized methods to construct pedigrees.  These are charts of genetic connections among individuals.  Pedigrees are analyzed to determine whether the inheritance of certain traits follows patterns through generations. The chart in the image is called a pedigree. It shows how the earlobe trait was passed from generation to generation within a family.

In a pedigree, squares symbolize males, and circles represent females. A horizontal line joining a male and female indicates that the
couple had offspring. Vertical lines indicate offspring which are listed left to right, in order of birth. Shading of the circle or square indicates an
individual who has the trait being traced. In this pedigree, the inheritance of the recessive trait is being traced. A is the dominant allele, and a is the recessive
allele. 

It is easy to study pea plants and fruit flies, but humans are not so easy. We select our own mates. We live in variable conditions and diverse environments. We reproduce when and if we want. Most people have small families, so it would be hard to make inferences about genetic probability.  Human subjects may live longer than the geneticist studying them, so tracking peoples genetics long term is more difficult than short lived organisms. Often genetic inheritance can be traced through a pedigree chart, helping Doctors determine its frequency in a population, what chromosomes genes may be linked to, and how mutations express themselves.  Below is a chart for polydactyly, extra fingers and toes. Ectrodactyly is missing fingers.  The gene for Polydactyly is rare but dominant.  There are places in the word where it is more common, and species where it is more common, such as cats. 

Pedigree charts have been widely used to study historical figures, as we can't test their DNA now, but we can search the records of history  to see which individuals in a family tree had the studied trait.  This has been done extensively with the Family Tree of Royals in Europe, England, and Russia. 

The case of the Blue Fugates

Methemoglobinemia occurs when red blood cells (RBCs) contain methemoglobin at levels higher than 1%. Methemoglobin results from the presence of iron in the oxidized ferric form (Fe³⁺) instead of the usual reduced ferrous form (Fe²⁺). This results in a decreased availability of oxygen to the tissues. This condition can be congenital or acquired. 

Symptoms are proportional to the methemoglobin level. At levels up to 20%, changes can occur in the color of blood and skin. For nearly 200 years, the Fugates — known as the blue people of Kentucky — remained largely sealed off from the outside world as they passed their blue skin from generation to generation. Unbeknownst to either of them, by some incalculable odds, both possessed a recessive gene that led to four of the seven children of this union being born with blue skin. In those days in rural eastern Kentucky, there were no roads, and a railroad wouldn’t even reach that part of the state until the early 1910s. As a result, many of the Fugates began to marry and have children within their own bloodline. This kind of genetic isolation allowed for the continued reproduction and expression of the Fugate family’s “blue skin” gene.

HUMAN GENETIC DISORDERS

The following genetic disorders have been studied with pedigree charts. 

Autosomal Recessive Inheritance

Albinism- the absence of pigment. 

This girl also has heterochromia- different colored eyes which is a genetic disorder, an autosomal dominant trait. 

 Sickle Cell Anemia- sickle shaped hemoglobin that clogs blood vessels. Jaundice, yellow skin,  is common because good blood cells break down more quickly than they should.  

Galactosemia- infants that can't digest breast milk. The enzymes that break down galactose- a sugar in breast milk, or lactose a sugar in formula, are not shaped correctly. It is life threatening if a baby has it causing cataracts, liver and kidney disease. It is simple to treat and diagnose.  A newborn baby is given a heal prick blood test right after they are born. They are given soy-based formula instead and a diet to avoid dairy. 

Phenylketonuria (PKU)- a rare inherited disorder that causes an amino acid called phenylalanine to build up in the body. PKU is caused by a defect in the gene that helps create the enzyme needed to break down phenylalanine. Again, diagnosed with a heal prick right at birth, the parents will be given information and counseling on a low protein diet. 

Autosomal Dominant Inheritance:

Achondroplasia- Dwarfism-a form of short-limbed dwarfism. The word achondroplasia literally means "without cartilage formation." Cartilage is a tough but flexible tissue that makes up much of the skeleton during early development. However, in achondroplasia the problem is not in forming cartilage but in converting it to bone (a process called ossification), particularly in the long bones of the arms and legs. Dwarfism is considered less than 4 ft 10 inches once adult stature is reached. 

X-linked Inheritance:

Hemophilia A- deficient blood clotting

Changes in Chromosome Structure:

Cri-du-chat- mental disability, skewed larynx- "cats cry"

Changes is Chromosome Number:

Downs Syndrome- Heart Defects, mental disability- #21 trisomy

Turners Syndrome- a condition that affects only females, results when one of the X chromosomes (sex chromosomes) is missing or partially missing. Turner syndrome can cause a variety of medical and developmental problems, including short height, failure of the ovaries to develop and heart defects.

Non-Mendelian Inheritance- Complex inheritance patterns
Of course human eyes do not come in multi-color, but they do come in many colors. How do eyes come in so many colors? That brings us to complex inheritance patterns, known as non-Mendelian inheritance. Many times inheritance is more complicated than the simple patterns observed by Mendel.
Each characteristic Mendel investigated was controlled by one gene that had two possible alleles, one of which was completely dominant to the other. This resulted in just two possible phenotypes for each characteristic. Each characteristic Mendel studied was also controlled by a gene on a different (nonhomologous) chromosome. As a result, each characteristic was inherited independently of the other characteristics. We now know that inheritance is often more complex than this. A characteristic may be controlled by one gene with two alleles, but the two alleles may have a different relationship than the simple dominant-recessive relationship that you have read about so far. For example, the two alleles may have a codominant or incompletely dominant relationship. For example, the two alleles may have a codominant or incompletely dominant relationship. Codominance-occurs when both alleles are expressed equally in the phenotype of the heterozygote. The red and white flower in the figure has codominant alleles for red petals and white petals. 

Codominance occurs when both alleles are expressed equally in the phenotype of the heterozygote.

Incomplete Dominance
Incomplete dominance occurs when the phenotype of the offspring is somewhere in between the phenotypes of both parents; a completely dominant allele
does not occur. For example, when red snapdragons are crossed with white snapdragons, the offspring are all pink. The pink color is an intermediate
between the two parent colors. When two plants with pink flowers are crossed, they will produce red, pink, and white flowers. The genotype of an organism with incomplete dominance can be determined from its phenotype.

Incomplete Dominance. The flower has pink petals because of incomplete dominance of a red-petal allele and a recessive white-petal allele. 

Barr Bodies:

While an female embryo is forming, one of the X-chromosomes is less active, essentially crumpled up ball called a Barr Body. Many of the genes are not used.  The level of gene activity produced by a single X chromosome is the normal "dosage" for a human. Men have this dosage because, well, they only have one X chromosome! Women have the same dosage for a different reason: they shut down one of their two X chromosomes in a process called X-inactivation. Which X is shut down is random. It could be the one the Father contributed, or it could be the one the Mother contributed, and it happens during the embryo stage of development.  All of the cells that the original cell divides into follow the same shut down pattern of the first cell........so if a cell made a barr body out of Moms X chromosome...all the decedents of that cell shut down Moms X.  A classic example of this is a calico cat.  All calico cats are female.  The cat is heterozygous for black and tan coat color.  They are considered a mosaic.  So are human females a mosaic.