Lethal alleles-meaning in genetics

Lethal alleles are alleles or a combination of alleles that cause the death of the organism that carries them. They are usually a result of mutations in genes that are essential for growth or development. Lethal alleles may be recessive, dominant, or conditional depending on the gene or genes involved. Example of lethal alleles are YY genotype in the color inheritance in mice. Yellow furs are dominant to grey. If a pair of yellow mice are mated, the results are always the same i.e. 2/3 of offsprings being yellow and 1/3 grey. They occur in a ratio of 2: 1 instead of 3: 1 because homozygous individuals for yellow die off before birth. i.e. genotype YY presents a lethal combination of genes. This has to been confirmed (i)  the present or dead embryo in the uterus (ii) the yellow mice do not breed true i.e., crossing yellow mice with yellow mice does not produce exclusively yellow mice. Please find free downloadable notes, exams and marking guides of agriculture, biology, Physics, chemistry etc.  from digitalteachers.co.ug website. Dr. Bbosa Science