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Evolution and natural selection
Independent assortment of genes and linkage
Mendelian genetics and punnett squares
Alagille syndrome (NORD)
Familial adenomatous polyposis
Multiple endocrine neoplasia
Polycystic kidney disease
Treacher Collins syndrome
von Hippel-Lindau disease
Gaucher disease (NORD)
Glycogen storage disease type I
Glycogen storage disease type II (NORD)
Glycogen storage disease type III
Glycogen storage disease type IV
Glycogen storage disease type V
Mucopolysaccharide storage disease type 1 (Hurler syndrome) (NORD)
Niemann-Pick disease type C
Niemann-Pick disease types A and B (NORD)
Primary ciliary dyskinesia
Sickle cell disease (NORD)
Tay-Sachs disease (NORD)
Cri du chat syndrome
Fragile X syndrome
Down syndrome (Trisomy 21)
Edwards syndrome (Trisomy 18)
Patau syndrome (Trisomy 13)
Fabry disease (NORD)
Glucose-6-phosphate dehydrogenase (G6PD) deficiency
Mucopolysaccharide storage disease type 2 (Hunter syndrome) (NORD)
Ornithine transcarbamylase deficiency
Autosomal trisomies: Pathology review
Miscellaneous genetic disorders: Pathology review
Muscular dystrophies and mitochondrial myopathies: Pathology review
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Inheritance is possible because of chromosomes.
These chromosomes come in pairs - one from mom and one from dad - so they’re called homologous chromosomes.
Each chromosome has genes, which are segments of DNA that carry genetic information for a specific trait.
And different versions of the same gene are called alleles.
As an example, brown eye color and blue eye color are both alleles for the eye color gene.
And each parent offers one allele of a gene.
Now, these alleles can be either dominant often represented with a capital letter, or recessive, represented with the corresponding lowercase letter, the difference being that it only takes one dominant allele for its traits to be expressed, whereas it takes two recessive alleles for its traits to be expressed.
Human somatic cells - that is, all of the cells aside from the sperm and eggs, which are called gametes - have 23 pairs of chromosomes; 22 somatic pairs and one sexual pair - adding up to 46 chromosomes in total.
These chromosomes, along with the alleles they carry, segregate during meiosis - which is the process of making new gametes.
Gametes only carry half the genetic information of the parent - so 23 chromosomes.
Once the male and female gametes merge during fertilization, their alleles combine to make the genotype —or genetic information— of the new organism.
For every gene, alleles can combine to give rise to three possible genotypes, homozygous dominant - or AA, heterozygous - or Aa - and homozygous recessive - or aa.
This determines all of a person’s features —or phenotype— such as eye color, hair color, or even whether or not they’re color blind.
Now, independent assortment means that no matter which alleles an organism inherits for one gene that codes for a trait like eye color, it won’t affect the alleles it inherits for another gene that codes for a different trait, like hair color.
Let’s start with a simple example. Let’s represent the eye color gene with the letter “a” and the hair color gene with the letter “d”.
Now, the dominant allele for eye color - A - stands for brown eyes and the recessive allele - a - stands for blue eyes.
On the other side, the dominant allele for hair color - D - stands for dark hair while the recessive allele - d - stands for blond hair.
So let’s say we have a person with a heterozygous genotype —Aa— for eye color and heterozygous genotype —Dd— for hair color.
This person would have the dominant allele features - so brown eyes and dark hair, even though they still carry a recessive allele for blond hair and blue eyes.
Genes are randomly assorted during the production of sperm and eggs. This is called independent assortment. This process is responsible for the different combination of genes that children receive from their parents.
If two genes are located close together on a chromosome, they are said to be linked. If these genes are passed on to a child, they will tend to be passed on together.
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