COMENTÁRIO DA 4ª QUESTÃO DE BIOLOGIA NO 8º ENCONTRO PAPA-ENEM
Autor: PAPA ENEM | Publicado: 2012-06-04T17:21:00-07:00

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O
heredograma abaixo representa o padrão de segregação para Acondroplasia, uma
das formas de nanismo humano, condicionada por um gene, que prejudica o
crescimento dos ossos durante o

desenvolvimento.
Essa doença genética humana apresenta letalidade, se ocorre em homozigose (AA).



(&nbsp;&nbsp;&nbsp; ) o padrão de herança do caráter em estudo
é autossômico dominante.

(&nbsp;&nbsp;&nbsp; )&nbsp; os indivíduos I1 e
II2 são homozigotos dominantes para o caráter.

(&nbsp;&nbsp;&nbsp; ) o casal III3 e III4 possui a probabilidade de
que, independente do sexo, ½ de seus filhos nascidos vivos possam ser normais e
1/2 portadores da doença.

(&nbsp;&nbsp;&nbsp; ) o indivíduo IV1 representa um aborto, e seu
genótipo é AA, que, em condição homozigótica, tem efeito tão severo que causa a
morte do portador ainda durante o desenvolvimento embrionário.

(&nbsp;&nbsp;&nbsp; ) todos os acondroplásicos nascidos vivos desta
genealogia são heterozigotos.


DADOS:
A Acondroplasia ou nanismo genético é um tipo de herança autossômica dominante,
e você pode perceber esta afirmação ainda no enunciado quando cita que a doença
é letal quando ocorre em homozigose(AA), então ter o fenótipo anão é ser
genotipicamente( Aa) enquanto o
fenótipo normal é manifestado pelo genótipo (aa).



Diante
da informação acima exposta e relacionando o 2º item para julgamento é
incorreta ou falsa já que os indivíduos I1 e II2 sendo anões só podem ter o
genótipo (Aa).



No
3º item para julgamento, afirma que o casal III3 e III4 possui a probabilidade
de que, independente do sexo, ½ de seus filhos nascidos vivos possam ser
normais e 1/2 portadores da doença. Tal item também é falso pois como seus
genótipos são:



Aa&nbsp; x&nbsp; Aa, podem gerar filhos vivos somente
na condição Aa Aa aa e para ter criança normais (aa) é uma chance em três,
portanto, 1/3 e para ter filhos anões (Aa) a chance é de duas em três,
portanto, 2/3.


O item 4 apresenta
uma afirmação correta, o que ratifica a informação prestada no enunciado e o
item 5 também apresenta uma afirmação correta, pois como já foi mencionado os
acondroplásicos vivos apresentam genótipo (Aa) portanto, heterozigotos.