Heredity Lesson 8

Levels of Heredity Gene level: SNPs, alleles Chromosome level: Trisomy, X-inactivation Cell level: Twins When we talk about heredity, someone might say, “You have your mother’s eyes.” What does that mean? Perhaps they are saying that the shape of your eyes resemble your mother. Or perhaps they are talking about the color of your eyes. At some level, we are the product of our genetics. We inherit our DNA from our mother and father and therefore express traits that resemble our mother, our father, or both depending on how the allele for each is expressed.

Alleles Remember: humans have 23 pairs of chromosomes From mother: chromosomes 1-22 and X From father: chromosomes 1-22 and X or Y Chromosome 1 from mom and chromosome 1 from dad form a homologous pair Homologous chromosomes contain the DNA for the same genes, but not necessarily the same alleles (versions of the genes)

Alleles The alleles you inherit determines your genotype Some alleles code for two different proteins: Dominant/recessive (brown/blue = brown eyes) Co-dominant (curly/straight = wavy hair) Incomplete dominance (purple/white = lavender) Some alleles code for non-functional or dysfunctional proteins How might an allele code for a non-functional or dysfunctional protein? Think back to the Central Dogma. Specifically, recall that tRNAs recognize specific codons, and tRNAs add amino acids to build proteins. And remember, for proteins, structure equals function. Protein structure comes from the sequence of amino acids, which is determined by the mRNA read by the ribosome. If a change to the DNA results in mRNA which now codes for an amino acid which dramatically alters the function of the protein the mRNA encodes, the function of the protein will be affected. The protein may not work as it normally would, causing a loss of function or a new unintended function.

Single Nucleotide Polymorphisms Single nucleotide polymorphisms (SNPs) Example: …ATTGTCCATGCCTGA… …ATTGTCTATGCCTGA… Why do SNPs matter? Might change one amino acid to another Might alter gene regulation Can be used as DNA fingerprints by forensic scientists SNPs occur on average every 300 bases in humans, which means each human genome contains nearly 10 million SNPs. A person’s unique combination of SNPs can be used to uniquely identify them, much like a fingerprint.

SNP Examples Sickle-Cell Anemia Cystic Fibrosis Muscular Dystrophy Some diseases are caused by a single SNP. Changing one nucleotide of DNA can have major effects depending on whether the DNA is regulatory, coding, or non-coding. Sickle-cell anemia, cystic fibrosis, and muscular dystrophy are examples of diseases that occur when a nucleotide within a coding region of DNA alters the amino acid sequence of the protein encoded by the gene. The resulting gene product, the protein, may not function “normally” resulting a new phenotype. Some mutations are harmful, while others confer a survival advantage. We will discuss the connection between DNA mutations and evolution in a future lesson. http://www.nature.com/scitable/topicpage/rare-genetic-disorders-learning-about-genetic-disease-979

Sickle-Cell Anemia Sickle Cell Disease: homozygous recessive Sickle Cell Trait: heterozygous People with cells that can sickle are protected against malaria, a disease spread by mosquitos which is common in Africa and the Mediterranean. Click the picture to play the video. Question: If Sickle Cell Disease is harmful, why would the allele continue to persist? Answer: because people with sickle cell trait (heterozygous for the harmful allele) are protected against malaria, a disease prevalent in certain parts of the world. It serves as a natural defense mechanism against malaria. The biological cost, having a child who is homozygous recessive and may die young from sickle cell trait, is an unfortunate consequence of malaria protection. Thankfully, new treatments are extending the lifespan of people with sickle cell disease and perhaps some day gene therapy will offer a cure to those affected.

Chromosomes What do you notice about this karyotype from a human cell? Trisomy 21 (3 copies of chromosome 21) and the cell comes from a male (XY).

Trisomy Trisomy means having an extra copy of a chromosome. Remember, chromosomes are extremely long stretches of DNA which code for 100’s and even 1000’s of genes. Having an extra copy of so many genes can have profound phenotypic consequences.

Trisomy 21 Trisomy 21 is commonly called Down Syndrome People with Down Syndrome have three copies of chromosome 21. People with Down Syndrome have features that appear different than people who have two copies of chromosome 21. Although rare, the odds have having a child with Down Syndrome increase in older mothers.

Down Syndrome Genotype Phenotype Click the pictures to play the videos. Genotype: What sort of cell division is occurring in the video? (Meiosis) How do you know? Phenotype: how is someone with Down Syndrome different from you? How are they the same? Genotype Phenotype

Twins Is it possible to have identical twins, where one twin is male and the other twin is female? Why or why not? How about fraternal (non-identical) twins?

Twins Identical Twins Fraternal Twins Why are the twins in the video considered “1 in a million”? Reflecting on what you have learned, will you think differently about the way you respond to people who appear different from you? Why or why not?

Mosaics and X-inactivation At what stage in embryonic development does X-inactivation occur? The Stem Cell stage (early in development) Does X-inactivation occur in males? Do any male cells lack an X chromosome? Are all females mosaics? (Yes) What does that mean?

Questions?